Top-levels FunctionDefinition to allow access to all function metadata (#686)

This top-levels `api.FunctionDefinition` which was formerly
experimental, and also adds import metadata to it. Now, it holds all
metadata known at compile time.

Here are the public API visible changes:
* api.ExportedFunction - replaced with api.FunctionDefinition as it is
  usable for all types of functions.
* api.Function - `.ParamTypes/ResultTypes()` are replaced with
  `.Definition().
* api.FunctionDefinition - extracted from experimental and adds
  `.Import()` to get the any imported module and function name.
* experimental.FunctionDefinition - replaced with
  api.FunctionDefinition.
* experimental.FunctionListenerFactory - adds first arg of the
  instantiated module name, as it can be different than compiled.
* wazero.CompiledModule - Adds `.ImportedFunctions()` and changes result
  type of `.ExportedFunctions()` to api.FunctionDefinition.

Internally, logic to create function definition are consolidated between
host and wasm-defined functions, notably wasm.Module now includes
`.BuildFunctionDefinitions()` which reduces duplication in
wasm.ModuleInstance `.BuildFunctions()`,

This obviates #681 by deleting the `ExportedFunction` type which
overlaps with this information.

This fixes #637 as it includes more metadata including imports.

Signed-off-by: Adrian Cole <adrian@tetrate.io>
Co-authored-by: Takeshi Yoneda <takeshi@tetrate.io>

runtime.go

@@ -176,6 +176,9 @@ func (r *runtime) CompileModule(ctx context.Context, binary []byte, cConfig Comp
 
 	internal.AssignModuleID(binary)
 
+	// Now that the module is validated, cache the function definitions.
+	internal.BuildFunctionDefinitions()
+
 	if err = r.store.Engine.CompileModule(ctx, internal); err != nil {
 		return nil, err
 	}