Use a fluid for the list of the reader's "hash procedures"

Use a fluid for the list of the reader's "hash procedures"

This allows customizing the reader behavior for a dynamic extent more easily.

* libguile/read.c (scm_read_hash_procedures): Renamed to
  `scm_i_read_hash_procedures'.
  (scm_i_read_hash_procedures_ref, scm_i_read_hash_procedures_set_x):
  New (internal) accessor functions for the fluid.
  (scm_read_hash_extend, scm_get_hash_procedure): Use these accessor
  functions.
  (scm_init_read): Create the fluid, named `%read-hash-procedures' instead of
  the previous plain list `read-hash-procedures'.

* test-suite/tests/reader.test: Adapt the "R6RS/SRFI-30 block comment
  syntax overridden" test to make use of the fluid.

* module/ice-9/deprecated.scm (read-hash-procedures):
  New identifier macro -- backward-compatibility shim.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>