Add ihex_rs_free() implementation.

Code shamelessly copied from mock_recordset_free() with a few
additional safety checks.
Add ihex_rs_free() to the public library API.
Use new ihex_rs_free() implementation in tests.
Add rudimentary unit test for ihex_rs_free.
Merge pull request #15 from acolomb/recordset-free

Implement ihex_rs_free()

Conflicts:
	tests/test_copy.c