Adds some helper methods to make using urtypes easier and cleaner

• Adds a urtype() classmethod to all registry items so that you can access the UR type name through the class:

> HDKey.urtype()
'crypto-hdkey'

• Adds a from_script classmethod to ScriptExpression so that premade script expressions can be loaded by their script name rather than UR ID

> ScriptExpression.from_script('wpkh')
ScriptExpression(404, 'wpkh')

• Adds a from_descriptor_key classmethod to HDKey that acts as the inverse of the existing descriptor_key method, enabling you to convert a bip32 key expression into an instance of HDKey

> HDKey.from_descriptor_key('[55f8fc5d/84h/1h/0h]tpubDCDuqu5HtBX2aD7wxvnHcj1DgFN1UVgzLkA1Ms4Va4P7TpJ3jDknkPLwWT2SqrKXNNAtJBCPcbJ8Tcpm6nLxgFapCZyhKgqwcEGv1BVpD7s')
HDKey(...)