Don't recreate the class

[jimmywarting]

V8 is unable to optimize this class if it's reconstructing it everytime.

ut_metadata/index.js

Lines 12 to 13 in b871aca

	module.exports = metadata => {
	class utMetadata extends EventEmitter {

• The class should be moved to the top level.
• A wire.ut_metadata instanceof check should work

Maybe should just get rid of the option to do: utMetadata(x) and instead use the wire.ut_metadata.setMetadata(x) method instead...