Consider if MrEnclave should be returned from ReportBody::mr_enclave()

[nick-mobilecoin]

Currently ReportBody::mr_enclave(), and ReportBody::mr_signer(), return Measurement.
This causes clients to ensure that they contain the proper types

        let Measurement::MrEnclave(mr_enclave) = report_body.mr_enclave() else {
            panic!("`mr_enclave()` should return a Measurement::MrEnclave");
        };

This isn't very ergonomic. It looks like during initial development of the core types Measurement was a generic wrapper around sgx_measurement_t and it was later updated to be an enum to prevent clients from mixing up MRENCLAVE and MRSIGNER values.

Suggest changing mr_enclave() and mr_signer() to return the specific types. It may be worth removing Measurement as well as it's probably not very useful as a container for either value.

[nick-mobilecoin]

Change in #288