add mapping and String() method for enum type in generated bindings

[rajatjindal]

this adds String() method for enum type as discussed in #127

If this approach looks ok, I will also try to add for variant and flag types.

[rajatjindal]

for variant lookup, it seems a bit odd:

e.g.

// NewTimestamp represents the variant "wasi:filesystem/types@0.2.0#new-timestamp".
//
// When setting a timestamp, this gives the value to set it to.
//
//	variant new-timestamp {
//		no-change,
//		now,
//		timestamp(datetime),
//	}

var mapNewTimestamp = map[uint8]string {
	0: "no-change",
	1: "now",
	2: "timestamp(datetime)",
}

func (self *NewTimestamp) String() string {
	return mapNewTimestamp[self.Tag()]
}

I am wondering if using String() method is a good choice for variant case as user might expect the string representation of actual value of NewTimestamp instead of just type of it.

[ydnar]

See comments

[ydnar]

Looking better! Just one small tweak.

I'd like to figure out how to add tests for this, but we can do that in a follow-up PR.

[ydnar]

Looks good. Some minor nits, but we can probably merge.