remove heap.[ch]

The heap implementation was not used for Strassen-Inline since
2005, and it was buggy anyway.

Additionally, the presence of "heap.c" in the directory was
confusing MakeMaker, potentially leading to make errors.
travis-ci: Inline::C problems solved by removing heap.c

The latest version may be used again.
update MANIFEST (removed heap* files)
Merge branch 'strassen-inline-heap-removal'