Allow users to redirect enclave standard input/output

[phlip9]

Hi all! This PR adds EnclaveBuilder::stdin, EnclaveBuilder::stdout, and EnclaveBuilder::stderr methods for optionally redirecting each stdio stream. My main use-case for this change is to do backtrace symbolizing in the enclave runner itself, rather than an outside wrapper script like intel-sgx/scripts/stack-trace-resolve. I found this to be cleaner; it keeps everything self-contained and the backtraces just come out exactly as you would expect.