Derive Debug trait for SGX types

### Background

When debugging, one usually needs to inspect the value of an object. For exactly this purpose, Rust offers the ability to implement or automatically derive Debug traits for user-defined types. Rust programmers have taken this godsend feature for granted.

### Problem

However, the SGX-related types defined in sgx_types crate do not have Debug trait implemented or derived. This makes it really cumbersome to debug values of SGX types. This absence of Debug trait for basic types also deviates the practice in Rust std crate, where (almost) all types support Debug traits. Moreover, due to Rust's the orphan rule, it is impossible for the users of Rust SGX SDK to manually implement Debug trait for SGX types defined in sgx_types crate.

### Feature Request

For the reasons above, I hope that Debug trait can be implemented (most can be derived automatically) for all SGX types defined in sgx_types. Thanks a lot.