Simplify and shrink reader buffers

Simplify and shrink reader buffers

A big on-stack buffer in a potentially long-running function can
interact badly with the GC's conservative scanning of the C stack.
It may make the scanning slower (since the stack frame is big) and
risks accidental retention of objects from stack detritus
(because the buffer isn't cleaned on entry).

* src/lread.c (stackbufsize): Remove.
(read_integer, read_string_literal, read_bool_vector): Use a local
buffer instead of piggy-backing on that in read0.
(read0): Reduce buffer to something suitable for most identifiers and
numbers.