Remove reflect from FastCall path

[antonmedv]

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Even if a function is defined in a fetcher, expr still used FetchFn and uses reflection unlike pure variables in a struct. FetchFn is guaranteed to use reflection.

func FetchFn(from interface{}, name string) reflect.Value {
	v := reflect.ValueOf(from)

	// Methods can be defined on any type.
	if v.NumMethod() > 0 {
		method := v.MethodByName(name)  //MethodByName utilizes reflection. Can this be avoided all together?
		if method.IsValid() {
			return method
		}
	}

Thank you.

Originally posted by @gitperson1980 in https://github.com/antonmedv/expr/issues/217#issuecomment-1011276835