Redesign nodeid format

First of all, it's now 40 bits instead of 48.

If the MSB of the first byte is 0, the remainder is a 39 bit siphash, just like before it was a 47 bit hash.

If it's a 1, the remaining bits are split up as so

Before

53M    examples/search.index
14M    build/aarch64-apple-darwin/doc/search.index
51M    build/aarch64-apple-darwin/compiler-doc/search.index

After

52M    examples/search.index
14M    build/aarch64-apple-darwin/doc/search.index
50M    build/aarch64-apple-darwin/compiler-doc/search.index

Description

1 0 0 N   N N N N   X X X X   X X ...
^ ^ ^ ~~~~~~~~~~~   ----------------- first row number
| | | |
| | | \------ "long" alphabitmap entry number
| | |
| | \-------- not a run
| |
| \--------- whole prefix match
|
\---------- inlined

1 0 1 N   N N N N   X X X X   X X ...
^ ^ ^ ~~~~~~~~~~~   ----------------- first row number
| | | |
| | | \------ run length - 1 (a single-element node is 00000)
| | |
| | \-------- run
| |
| \--------- suffix-only
|
\---------- inlined

1 1 0 N   N N N N   X X X X   X X ...
^ ^ ^ ~~~~~~~~~~~   ----------------- first row number
| | | |
| | | \------ data length - 1
| | |
| | \-------- not a run
| |
| \--------- suffix-only
|
\---------- inlined

1 1 1 N   N N N N   X X X X   X X ...
^ ^ ^ ~~~~~~~~~~~   ----------------- first row number
| | | |
| | | \------ run length - 1
| | |
| | \-------- run
| |
| \--------- suffix-only
|
\---------- inlined