Getters/Setters for fields

[c410-f3r]

Fixes #222.

[RReverser]

Is it possible to eliminate the extra method on the JS side and keep only the getter?

[c410-f3r]

@RReverser Sure. I was a bit apprehensive about this aspect but I finally came up with the following rules.

Input			Output
js_name attr	getter/setter with name	getter/setter w/o name	get/set method name	ordinary method name
-	-	-	n/a	fn name
-	-	X	fn name	n/a
-	X	-	getter/setter name	n/a
-	X	X	forbidden¹	n/a
X	-	-	n/a	js_name
X	-	X	js_name	ignored²
X	X	-	getter/setter name	ignored²
X	X	X	forbidden¹	ignored²

¹ Duplicated getters or duplicated setters
² Ordinary method is ignored when the js_name attribute is used together with the getter/setter attribute

[RReverser]

Ah, so this extra method is generated only due to js_name in the example above and would be omitted otherwise? That makes sense.

[alexcrichton]

Sorry for the delay in review, I didn't realize this was ready! Feel free to ping whenever it's ready and we can take a look

[alexcrichton]

Could this return an error for unsupported OperationKind types

[c410-f3r]

Should an error be thrown? This line code roughly translates to: Don't do fancy things for anything that isn't a getter or a setter.

[alexcrichton]

Ah yeah I think we should generate an error here because something like #[wasm_bindgen(indexing_getter)] (or something like that) isn't fully supported here

[alexcrichton]

Thanks again for this @c410-f3r!