guile/test-suite/tests/c-api

This directory contains tests for Guile's C API.  At the moment, the
test suite doesn't have any way to run these automatically --- we need
to 1) figure out how to run the compiler, and 2) figure out how to
integrate results from C tests into the test suite statistics.

Nonetheless, it's better to have this code accumulating here than
someplace else where nobody can find it.