* 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-42/ec.scm: New file; reference implementation of
SRFI 42.
* module/srfi/srfi-42.scm: New file; module for SRFI 42.
* module/Makefile.am (SRFI_SOURCES): Add srfi/srfi-42.scm.
(NOCOMP_SOURCES): Add srfi/srfi-42/ec.scm.
* test-suite/tests/srfi-42.test: New file; test suite for SRFI 42.
* test-suite/Makefile.am: SCM_TESTS: Add tests/srfi-42.test.
