Add implementation of SRFI 42

Add implementation of SRFI 42

* 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.