Split libvcx crate

[Patrik-Stas]

Split libvcx in 2 crates

• libvcx_core - this would be today's "handle" layer of libvcx
• libvcx - this would be today's api_c later of libvcx, eg built on top of libvcx_core crate

This will enable building different FFI approaches on shared base layer, ruling out a chance of dealing with some c symbol conflicts. For example we can build new libvcx_napi on top of libvcx_core as initiated in this PR #665
Similarly per @dinbtechit question #669 this would enable writing new wrappers, like flutter, with relative ease.

Implementation of this should have no breaking changes and the split should be invisible to the currently built libvcx dynamic library

[Patrik-Stas]

libvcx was not split, but separated in 2 submodules api_c, api_vcx, see #710