Add implementation of SRFI 45

Add implementation of SRFI 45

* module/srfi/srfi-45.scm: New file, containing the reference implementation of
  SRFI 45, slightly adapted to use SRFI-9.
* module/Makefile.am (SRFI_SOURCES): Added srfi/srfi-45.scm.

* test-suite/tests/srfi-45.test: New file.
* test-suite/Makefile.am (SCM_TESTS): Add tests/srfi-45.test.

* doc/ref/srfi-modules.texi (SRFI-45): New node and subsection;
  essentially a shortended transcript of the SRFI-45 specification.