breaking: Encode/decode all enums by value.

[jcrist]

Previously integer enums encoded/decoded by their integer values,
while all other enums encoded/decoded by their string names. This PR
changes things so that all enums encode/decode by their values (string
and integer values only supported for now).

This is a breaking change

Before this PR:

import enum
import msgspec

class Example(enum.Enum):
    A = "x"

msgspec.json.encode(Example.A)
# > b'"A"'

After this PR:

import enum
import msgspec

class Example(enum.Enum):
    A = "x"

msgspec.json.encode(Example.A)
# > b'"x"'

Many other tools in the python ecosystem support encoding enums as their
values by default (orjson, pydantic, cattrs, json), but no tool
AFAICT supports encoding as their names by default. The purpose of this
change is to better match the rest of the Python ecosystem.

Fixes #209.