* module/rnrs/arithmetic/6/fixnums.scm: Fix missing imports;
(fixnum-width, greatest-fixnum, least-fixnum): Redefine these as
zero-argument procedures; Fix argument mismatches in several functions.
* test-suite/Makefile.am: Add tests/r6rs-arithmetic-fixnums.test to
SCM_TESTS.
* test-suite/tests/r6rs-arithmetic-fixnums.test: New file.
