lisp: Speed up parsing identifiers.
lisp: Speed up parsing identifiers.

This and the previous two patches reduce run time for the two
`allocate_*_garbage` benchmarks by about 18%. (This is because a lot of
the time is spent parsing and initializing psyntax.)

Right now, parsing is roughly
* 2 parts skipping whitespace (and comments)
* 1 part other lexing
* 1 part parsing
* 1 part interning symbols (and strings)
* several parts GC

jorendorff/cell-gc,jorendorff/cell-gc