Refactor buffering code in the Lisp reader

Refactor buffering code in the Lisp reader

Abstract the buffering in the reader with a struct and plain functions
instead of rather unhygienic preprocessor macros.

* src/lread.c (READ_AND_BUFFER, INVALID_SYNTAX_WITH_BUFFER):
Removed, replaced with...
(readbuf_t, readbuf_grow, add_char_to_buffer, read_and_buffer)
(invalid_syntax_with_buffer): ...these new functions and struct.
(read0): Use new code instead of old preprocessor macros.

(cherry picked from commit 32a0367554334c14fc1a9242fa6bfc45abe3d032)