consolidate make-covenant functions and implement createrawtransaction with covenants

[pinheadmz]

I think all the covenant-filling logic should be consolidated into either output.js or covenant.js and exported as library functions. Currently the best way just to find out what goes into a covenant is in the wallet make___() functions:

e.g. for OPENs:

hsd/lib/wallet/wallet.js

Lines 1613 to 1619 in 98a6491

	const output = new Output();
	output.address = addr;
	output.value = 0;
	output.covenant.type = types.OPEN;
	output.covenant.pushHash(nameHash);
	output.covenant.pushU32(0);
	output.covenant.push(rawName);

This will make it a lot easier to write tx-building code with the hsd library, and it should also allow us to implement a JSON api like createrawtransaction where a custom combination of covenants are included.

See #465