Add msgspec.to_builtins method

[jcrist]

This adds a new msgspec.to_builtins function, for transforming objects composed of all the types msgspec supports to objects composed only of types standard python serializers support (list, tuple, int, str, ...).

While this method may be useful directly to users, the primary intended use is for wrapping it with additional serialization APIs within msgspec itself (for example, a new msgspec.yaml module). While JSON may require high-performance and a builtin encoder, config-files like YAML, TOML, ... usually aren't perforamnce sensitive, so the extra copies here shouldn't matter.

Once this is in, we'll follow up with a msgspec.from_builtins for handling the "decoding" side of things.

[jcrist]

Example of usage:

In [1]: import msgspec                         

In [2]: class User(msgspec.Struct):
   ...:     name: str                          
   ...:     groups: set[str] = set()
   ...:     email: str | None = None
   ...:                                        

In [3]: alice = User("alice", groups={"admin", "engineering"}, email="alice@company.com")

In [4]: alice                                  
Out[4]: User(name='alice', groups={'engineering', 'admin'}, email='alice@company.com')

In [5]: msg = msgspec.to_builtins(alice)  # convert to simple types

In [6]: msg                                    
Out[6]:                                        
{'name': 'alice',                              
 'groups': ['engineering', 'admin'],
 'email': 'alice@company.com'}

In [7]: import yaml                            

In [8]: print(yaml.safe_dump(msg, sort_keys=False))  # can pass to downstream libraries
name: alice                                    
groups:                                        
- engineering                                  
- admin                                        
email: alice@company.com

Like I said above, ideally most users won't ever need to directly access this API - we should instead use it to wrap common downstream libraries (yaml, toml, ...) in high-level APIs mirroring the existing msgspec.json/msgspec.msgpack modules.