Adds ResultNames to HostFunctionBuilder and FunctionDefinition (#887)

This adds ResultNames to HostFunctionBuilder and FunctionDefinition
which helps for multi-results or special-cased ones.

End users can access result names in `FunctionDefinition.ResultNames` or
set for their own host functions via
`HostFunctionBuilder.WithResultNames`. This change adds them for all
built-in functions where result names help.

Most notably, GOOS=js uses `ProxyFunc` to allow logging when a function
returns multiple results. Before, the results were returned without
names: e.g. `11231,1` and now they are named like `n=11231,ok=1`.

We soon plan to allow more visibility in WASI, for example, logging
results that will write to memory offsets. This infrastructure makes it
possible to do that.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

api/wasm.go

@@ -278,6 +278,10 @@ type FunctionDefinition interface {
 	//
 	// See ValueType documentation for encoding rules.
 	ResultTypes() []ValueType
+
+	// ResultNames are index-correlated with ResultTypes or nil if not
+	// available for one or more results.
+	ResultNames() []string
 }
 
 // Function is a WebAssembly function exported from an instantiated module