We can concoct any scheme we like for mapping
between uintptrs and mutexes.

Instead of using unsafe to map between actual pointers and uintptrs,
use a simple numbering scheme. Zero means nil. The next 12 ints
correspond to sqlite's 12 built-in mutexes. Everything after that
is an index into the mutex pool, appropriately shifted by 12.

This makes vet happy and makes the pointer checker happy.

It adds instructions and branches to the mutex acquisition path,
which is definitely unfortunate. In exchange, though,
we get simplicity, safety, and easy to reason about mutex "pointers".

This further simplifies the code.
It also removes a branch from the mutex lookup code,
at the code of adding bounds checks to the static mutex lookups.

lib: use less unsafe for mutexes

Closes #123

See merge request !49

A+C: add myself

Closes #124

See merge request !50

Only add the `beginMode`, set by `_txlock`, after BEGIN in non-readonly
transactions. Use the `ReadOnly` member of the `driver.TxOptions`
parameter to determine when and when not to add the `beginMode`.

sqlite: only add tx_lock begin directive for write txs

Closes #127

See merge request !52

Export Sqlite3_index* structs

Closes #134

See merge request !53

Does not fail for 1000 cases without -race, does fail with -race.

jnml@3900x:~/src/modernc.org/sqlite$ go test -v -timeout 24h -run TestIssue142 -race |& tee log-test-issue142
test binary compiled for linux/amd64
=== RUN   TestIssue142
    all_test.go:3406: 0
    all_test.go:3406: 1
    all_test.go:3406: 2
    all_test.go:3406: 3
    all_test.go:3406: 4
    all_test.go:3521: database is locked (5) (SQLITE_BUSY)
--- FAIL: TestIssue142 (0.10s)
FAIL
exit status 1
FAIL	modernc.org/sqlite	0.110s
jnml@3900x:~/src/modernc.org/sqlite$

This is basically the same as the one from zombiezen.com/go/sqlite, but
only supports scalar functions for now.

This allows registering custom aggregate and window functions (which
provide Step, Inverse, Value, and Final callbacks). It uses basically
the same interface as the zombiezen.com/go/sqlite package, which already
handles some trickier cases around multiple or concurrent invocations of
the same function.

This ensures that they cannot be GCed.

See cznic/sqlite!54 (comment 1396299066)
for more details.

Allow registering aggregate functions

See merge request cznic/sqlite!54

Closes #148.

Fix a data race in `lib/mutex.go`

Closes #148

See merge request !55

Add API to sqlite3_serialize/deserialize and sqlite3_backup_*

See merge request cznic/sqlite!56

=== RUN   TestIssue152
=== RUN   TestIssue152/SELECT_1_WHERE_false
=== RUN   TestIssue152/--_just_a_comment
    all_test.go:3019: sql: no Rows available
--- FAIL: TestIssue152 (0.00s)
    --- PASS: TestIssue152/SELECT_1_WHERE_false (0.00s)
    --- FAIL: TestIssue152/--_just_a_comment (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x5b5c69]

goroutine 9 [running]:
testing.tRunner.func1.2({0xaeff40, 0x13d9870})
	/usr/local/go/src/testing/testing.go:1526 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1529 +0x39f
panic({0xaeff40, 0x13d9870})
	/usr/local/go/src/runtime/panic.go:884 +0x213
database/sql.(*Rows).close.func1()
	/usr/local/go/src/database/sql/sql.go:3287 +0x29
database/sql.withLock({0xf82190, 0xc0001be990}, 0xc0001fbca0)
	/usr/local/go/src/database/sql/sql.go:3405 +0x8c
database/sql.(*Rows).close(0xc000133480, {0x0, 0x0})
	/usr/local/go/src/database/sql/sql.go:3286 +0x16d
database/sql.(*Rows).Close(0x4f12c6?)
	/usr/local/go/src/database/sql/sql.go:3270 +0x1d
runtime.Goexit()
	/usr/local/go/src/runtime/panic.go:522 +0x177
testing.(*common).FailNow(0xc000135040)
	/usr/local/go/src/testing/testing.go:980 +0x4c
testing.(*common).Fatal(0xc000135040, {0xc0001fbf18?, 0xc00001a0f0?, 0xb34fee?})
	/usr/local/go/src/testing/testing.go:1057 +0x58
modernc.org/sqlite.testIssue152(0xc000135040, {0xb34fee, 0x11})
	/home/jnml/src/modernc.org/sqlite/all_test.go:3019 +0x1d0
modernc.org/sqlite.TestIssue152.func1(0x0?)
	/home/jnml/src/modernc.org/sqlite/all_test.go:2998 +0x25
testing.tRunner(0xc000135040, 0xc0000638a0)
	/usr/local/go/src/testing/testing.go:1576 +0x10b
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1629 +0x3ea
exit status 2
FAIL	modernc.org/sqlite	0.008s

test binary compiled for linux/amd64
=== RUN   TestIssue153
=== RUN   TestIssue153/SELECT_1_WHERE_false
=== RUN   TestIssue153/--_just_a_comment
=== RUN   TestIssue153/#00
--- PASS: TestIssue153 (0.00s)
    --- PASS: TestIssue153/SELECT_1_WHERE_false (0.00s)
    --- PASS: TestIssue153/--_just_a_comment (0.00s)
    --- PASS: TestIssue153/#00 (0.00s)
PASS
ok  	modernc.org/sqlite	0.029s

From the issue:

> This issue might be related to this one: #152 (closed)

Confirmed.

Previously ColumnTypeScanType returned the reflect.Type of [][]byte
for BLOB columns, resulting in a failure to scan when the type was
constructed dynamically based on this return.

Update CONTRIBUTORS as requested

See merge request cznic/sqlite!60

Update sqlite.go's docs for Driver.Open()

See merge request cznic/sqlite!59

Implements `RegisterCollationUtf8` and `MustRegisterCollationUtf8`

Fixes #163

See merge request cznic/sqlite!61