Reduce duplication of code in provider_api.rs

[vn971]

Currently, the code in rust-lang/sys/sgx/abi/usercalls/mod.rs and in rust-sgx/intel-sgx/async-usercalls/src/provider_api.rs (#515) is almost identical for functions copy_user_buffer and string_from_bytebuffer.
After the above PR gets merged, we may be interested in using the new code from inside the Rust standard library (official rust-lang repository). Note that simply making the function public may not be desirable because the code can be considered internal to the SGX platform. This decision is up for the person to pick up this issue