[MerlinC] Supprt digitrec/smith_waterman/fft

[comaniac]

• Add support to the three samples.
• Add simple unit tests for Merlin C and C backend.
• Reorg samples.

Note that the way I reorged samples is making a duplication for Merlin backend, but most of them, especially the HeteroCL kernel part, should not be duplicated. You're welcome to change the structure later on.

[comaniac]

hmm...it turns out that the way I set up the unit test was not the pytest style. I'll fix it soon.