In this PR I re-factor and do some clean-up of the code running outside the enclave ({src,include}/enclave/outside).

Changelog:

• Re-factor SGXWamrWasmModule.{cpp,h} toEnclaveInterface.{cpp,h}: as discussed offline, the real WASM module is inside the enclave, so this name was misleading.
• Re-factor SGXWAMRWasmModule.S to getSgxSupport.S: the name for the ASM script was also very misleading. This file checks for HW support for SGX. There are other ways to do it (not necessarily less involved), but in any case matter for a different PR.
• Factor-out the set of defined ECalls from EnclaveInterface.h to ecalls.h for clarity. Note that ECalls are declared outside the enclave, but defined inside*.
• Add method to process errors when doing ECalls. This was verbose and repeated before as we need to check for two error codes (one for SGX-system errors, one for Faasm-logic errors inside the enclave).

*This is not entirely true, as SGX generates stubbed calls using the EDGER8R tool, so a sort of ECall is defined outside the enclave as part of the build process. But this is not relevant to the discussion.