Performance improvement: synchronous usercalls inside async-usercalls methods

[vn971]

Currently, there are usages of synchronous usercalls in the async-usercalls module (to be merged to master with PR https://github.com/fortanix/rust-sgx/pull/515).

This might be problematic for performance. Performance impact of said synchronous usercalls needs to be measured and/or fixed by converting them to async-usercalls.

Currently known instances of this behaviour (sometimes implicit, sometimes explicit):

• intel-sgx/async-usercalls/src/provider_api.rs, line 32, impl AsyncUsercallProvider, pub fn read
• intel-sgx/async-usercalls/src/provider_api.rs, User::<[u8]>::uninitialized leads to a synchronous usercall
• intel-sgx/async-usercalls/src/provider_api.rs, line 105, function bind_stream, sync usercall upon being dropped
• intel-sgx/async-usercalls/src/provider_api.rs, line 104, _addr_buf is of type MakeSend<User<[u8]>> which will lead to a synchronous usercall upon being dropped
• potentially other places as well. This task should be re-evaluated upon picking up, it is possible that we may want to start with performance measurements first, especially for the less obvious places.