Add support for R6RS/SRFI-30 nested block comments.

Add support for R6RS/SRFI-30 nested block comments.

Suggested by Andreas Rottmann <a.rottmann@gmx.at>.

* libguile/read.c (flush_ws, scm_read_sharp): Add support for
  R6RS/SRFI-30 block comments.
  (scm_read_r6rs_block_comment): New function.

* test-suite/tests/reader.test (exception:unterminated-block-comment):
  Adjust to match both block comment styles.
  ("reading")["R6RS/SRFI-30 block comment", "R6RS/SRFI-30 nested block
  comment", "R6RS/SRFI-30 block comment syntax overridden"]: New tests.
  ("exceptions")["R6RS/SRFI-30 unterminated nested block comment"]: New
  test.

* doc/ref/api-evaluation.texi (Block Comments): Mention SRFI-30/R6RS
  block comments.

* doc/ref/srfi-modules.texi (SRFI-30): New node.