Add an Instance method to lookup an exported func (bytecodealliance#69)

This avoid having to check both the return from `.GetExport()` and
`.Func()`, and follows after `instance.get_func()` from the Rust API.

instance.go

@@ -135,6 +135,18 @@ func (i *Instance) GetExport(name string) *Extern {
 	return i.exports[name]
 }
 
+// GetFunc attemps to find a function on this instance by `name`.
+//
+// May return `nil` if this instance has no function named `name`,
+// it is not a function, etc.
+func (i *Instance) GetFunc(name string) *Func {
+	f := i.GetExport(name)
+	if f == nil {
+		return nil
+	}
+	return f.Func()
+}
+
 func (i *Instance) populateExports() {
 	exports := i.Exports()
 	for j, ty := range i.Type().Exports() {

instance_test.go

@@ -115,3 +115,43 @@ func TestInstanceBad(t *testing.T) {
 		panic("expected an error")
 	}
 }
+
+func TestInstanceGetFunc(t *testing.T) {
+	wasm, err := Wat2Wasm(`
+          (module
+            (func (export "f") (nop))
+            (global (export "g") i32 (i32.const 0))
+          )
+	`)
+	if err != nil {
+		panic(err)
+	}
+	store := NewStore(NewEngine())
+	module, err := NewModule(store.Engine, wasm)
+	if err != nil {
+		panic(err)
+	}
+	instance, err := NewInstance(store, module, []*Extern{})
+	if err != nil {
+		panic(err)
+	}
+
+	f := instance.GetFunc("f")
+	if f == nil {
+		panic("expected a function")
+	}
+	_, err = f.Call()
+	if err != nil {
+		panic(err)
+	}
+
+	f = instance.GetFunc("g")
+	if f != nil {
+		panic("expected an error")
+	}
+
+	f = instance.GetFunc("f2")
+	if f != nil {
+		panic("expected an error")
+	}
+}