Descriptor key parsing and helper methods

[ghost]

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(...)

[github-actions]

✅ Result of Pytest Coverage

---------- coverage: platform linux, python 3.9.10-final-0 -----------

Name	Stmts	Miss	Cover
src/urtypes/init.py	2	0	100%
src/urtypes/bytes.py	16	1	94%
src/urtypes/cbor/init.py	3	0	100%
src/urtypes/cbor/data.py	30	4	87%
src/urtypes/cbor/decoder.py	132	58	56%
src/urtypes/cbor/encoder.py	94	20	79%
src/urtypes/crypto/init.py	9	0	100%
src/urtypes/crypto/account.py	26	1	96%
src/urtypes/crypto/bip39.py	23	1	96%
src/urtypes/crypto/coin_info.py	25	1	96%
src/urtypes/crypto/ec_key.py	31	1	97%
src/urtypes/crypto/hd_key.py	208	10	95%
src/urtypes/crypto/keypath.py	55	1	98%
src/urtypes/crypto/multi_key.py	37	0	100%
src/urtypes/crypto/output.py	127	17	87%
src/urtypes/crypto/psbt.py	6	1	83%
src/urtypes/registry.py	35	4	89%
TOTAL	859	120	86%

~ 15 passed in 0.25s ~