* module/ice-9/boot-9.scm (%cond-expand-features): Remove redundant list
of feature identifiers in the comment. Explain more clearly what
belongs in this list. Remove srfi-6.
* module/srfi/srfi-4.scm, module/srfi/srfi-27.scm,
module/srfi/srfi-31.scm, module/srfi/srfi-38.scm,
module/srfi/srfi-39.scm, module/srfi/srfi-42.scm,
module/srfi/srfi-45.scm, module/srfi/srfi-67.scm: Add missing
'cond-expand-provide'.
* module/srfi/srfi-69.scm: Fix erroneous 'cond-expand-provide'.
* doc/ref/srfi-modules.texi (SRFI-0): Update the list of features in
Guile core.
* module/srfi/srfi-27.scm: New file; implementation of SRFI 27 in terms
of the existing random number generator.
* module/Makefile.am (SRFI_SOURCES): Add srfi/srfi-27.scm.
* test-suite/tests/srfi-27.test: New file; minimal test suite for SRFI 27.
* test-suite/Makefile.am (SCM_TESTS): Add tests/srfi-27.test.
* doc/ref/srfi-modules.texi: Add subsection on SRFI-27 based
on the specification.
