Add support for renaming fields - #98

In [1]: import msgspec

In [2]: class Example(msgspec.Struct, rename="camel"):
   ...:     field_one: int
   ...:     field_two: str
   ...: 

In [3]: ex = Example(1, field_two="two")  # in python code the field names are the same

In [4]: ex
Out[4]: Example(field_one=1, field_two='two')

In [5]: s = msgspec.json.encode(ex)  # The renamed fields are used when encoding/decoding

In [6]: s
Out[6]: b'{"fieldOne":1,"fieldTwo":"two"}'

In [7]: msgspec.json.decode(s, type=Example)
Out[7]: Example(field_one=1, field_two='two')

In [8]: msgspec.json.decode(b'{"fieldOne":1}', type=Example)  # The renamed fields are also used in DecodeError messages
---------------------------------------------------------------------------
DecodeError                               Traceback (most recent call last)
Input In [8], in <cell line: 1>()
----> 1 msgspec.json.decode(b'{"fieldOne":1}', type=Example)

DecodeError: Object missing required field `fieldTwo`