Investigate making all enclaves use one cargo workspace

[cbeck88]

Currently our build has 5 cargo workspaces:

• Untrusted code
• Consensus enclave
• Fog ingest enclave
• Fog view enclave
• Fog ledger enclave

This was expedient for getting the cargo-based enclave build to work. However, there is not much reason for each enclave to have a separate workspace. That creates some problems:

• Separate Cargo.toml maintained with patches 5 times
• Separate Cargo.lock files and dependabot PRs for 5 different work spaces
• Many targets are built 5 times because there is a different target directory per workspace. For instance I think we build rust-mbedtls 5 times when we do a release.

The enclaves cannot be in the same workspace as untrusted, because untrusted turns on std features for many crates and pulls in the standard library, but this does not build in the enclave.

However, we may be able to unify the four enclave workspace into one workspace for all enclaves. Then we could have just two workspaces overall instead of five. The main reason would be to make the build faster and ease maintenance.

[nick-mobilecoin]

Documenting here for later perusal.
As part of the #1062 work looked into per-package-target. The intent was to try and resolve the multiple workspaces into one.
It seems that the per-pakcage-target is still a bit immature.

See rust-lang/cargo#9521, rust-lang/cargo#9406, and rust-lang/cargo#9451
The enclaves are utilizing zbuild-std