This allows the environment to control the signatures used for direct
function calls. The signature and calling convention may depend on
whether the function is imported or local.
Also add WasmRuntime::declare_func_{import,type} to notify the runtime
about imported and local functions. This is necessary so the runtime
knows what function indexes are referring to .
Since imported and local functions are now declared to the runtime, it
is no longer necessary to return hashes mapping between WebAssembly
indexes and Cretonne entities.
Also stop return null entries for the imported functions in the
TranslationResult. Just return a vector of local functions.
The function environment is now expected to keep track of the function
signatures in the module, and it is asked to generate Cretonne
signatures to be used for indirect calls.
The combination of make_indirect_sig() and translate_call_indirect()
callbacks allow the runtime to insert additional function arguments for
indirect calls such as vmctx pointers and CFI-style signature identifiers.
