Tune reading of radix integers

Tune reading of radix integers

This improves the performance of (read "%xFF") by about 25%
on my platform.
* src/lread.c: Include <vla.h>, so that we can better document
buffer sizes of arguments.
(invalid_radix_integer_format, stackbufsize): New constants.
(free_contents): Remove.  All uses removed.
(invalid_radix_integer): New function.
(read_integer): New arg STACKBUF.  Assume radix is in range.
All uses changed.  Use STACKBUF to avoid calling malloc in the
usual case.  Use grow_read_buffer to simplify.
(read1): Tune.  Improve quality of diagnostic when
MOST_POSITIVE_FIXNUM < radix <= EMACS_INT_MAX.