Run crypto functions in SGX

When CodeChain executes sign function, a private key may be exposed in memory. If we run the code in SGX, other programs cannot find the private key value in memory.

The issue names CodeChain's sign function but no file, test, or entry point. Start by locating that function and checking how cryptographic execution and SGX support are structured; done means the signing code runs inside SGX so other programs cannot read the private key from memory.

Written by the indexing model from the issue text.