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 !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 cznic/sqlite!55