Create a crate or set of crates to build enclaves

[nick-mobilecoin]

Currently the enclave building logic lives in the mobilecoin repo at util/build/enclave/src/lib.rs.
Similar logic is also repeated in the test_enclave for the untrusted interface. A common set of logic and crate(s) should be available in this repo for consumers to use to build, and sign, enclaves.