Refactor: Namespace cleansing ⚠️

[Shdorsh]

🔲Depends on #173

⚠️Might break projects that rely on standard libraries linked via mpc.h

Cleans the headspace by:

1. Creating a private header internals/mpc_internal.h, which integrates mpc.h.
2. Hides dependencies in mpc_internal.h.
3. Removes unused dependencies.

This here also tags the release as "v0.10". Starting now, releases should be tagged.