Wrap MastForest in Program and Library in Arc

[bobbinth]

This PR wraps MastForest fields in Program and Library structs in Arc. This way, cloning these objects becomes a relatively cheap operation.

Additionally, this PR contains the following changes/refactorings:

1. A check was added to make sure that an entrypoint of a Program is registered as a root in the MastForest. This was already happening - but now we have an explicit check for this.
2. For every re-exported procedure, we now add an External node to the MastForest. This makes Library and Program structs a bit more consistent and also removes a somewhat un-intuitive situations where if a library had only re-exported procedures, its MastForest would be completely empty. Since now there is an easy way to track re-exported procedures in the MastForest itself, the distinction between local and re-exported procedures was removed from the Library struct (but I added a method that allows checking if a given export is a re-export).
3. In the MastForestBuilder, we now insert Procedure structs for re-exported procedures too. This allowed removing procedure_hashes map and unify the way we treat local and re-exported procedures.

There is one thing we can improve in the future: if we re-export a procedure from the library and a procedure with the same digest is locally defined, we may end up with some unreachable nodes in the MAST forest. This is technically not a bug - but it does create bloat in the MAST forest. In general, I think if a library has a locally-defined procedure and also re-exports a procedure with the same root, the re-exported procedure should take precedence.

This PR closes #1430. As suggested by @bitwalker, I did not to add Version to the Library. We should probably either remove Version entirely or move it to miden-core. But I left this for future PRs.

[plafer]

LGTM! Left a few nits