Added r7rs test suite

r7rs-tests.scm is taken from Chibi, which is read from r7rs.scm.
we haven't added it to TESTS yet, for some tests fails because
of the following reasons:

- Missing exact complex number support.  Most of failures caused
  by this (including number writing tests).
- Behavior of default 'write' (should recognize circular but not
  shared).
- (features) doesn't have r7rs.  We add it once we complete r7rs
  features.

To run r7rs test, go src and run ./gosh -ftest ../test/r7rs