Use `std`-aware cargo instead of `sgx_tstd` / xargo

This is mostly a heads up that design work is underway to allow parts of `std` to be swappable via cargo:

https://internals.rust-lang.org/t/pre-rfc-std-aware-cargo/9450

Would be good to make sure this feature meets the needs of rust-sgx-sdk

Read the linked std-aware-cargo pre-RFC first, then compare its proposed capabilities with the SDK's current use of sgx_tstd and xargo. The issue does not name files, tests, or a concrete acceptance criterion; done would require establishing and implementing the compatibility needs for rust-sgx-sdk.

Written by the indexing model from the issue text.