Fix non-struct types with methods on them

[matthewmueller]

Right now this will panic:

package main

import "fmt"

// Text fn
type Text string

func (t Text) String() string {
    return string(t)
}

func main() {
    fmt.Println(Text("hi world").String())
}

I made a early assumption that all method receivers are structs. Need to back out of this now, so method.Recv() returns a Typer. More specifically, Also need to change it so Structer is type of Typer and update the breakage there:

var _ Typer = (*structdef)(nil)

Inside:

• https://github.com/matthewmueller/joy/blob/master/internal/compiler/defs/struct.go#L27