Add wasmer_version to the C-API

[syrusakbary]

Add a wasmer_version function to Wasmer C API.
It will also help to do sanity checks.

int wasmer_version(void)

Other functions that could be useful:

• wasmer_available_compilers
• wasmer_available_engines

[MarkMcCaskey]

Adding onto this issue, perhaps we want to use an enum for this purpose.

We should discuss the future of the C API and figure out what pieces are missing.

[syrusakbary]

I think versions are normally always reported as strings, I think that's more convenient as enums.

But yeah, for compilers and engines enums is the right approach!