Reflection guided serialization for rbx_xml

[LPGhatguy]

Fixes #18, fixes #16, fixes #14.

Converts rbx_xml to depend on rbx_reflection, using its database to drive serialization. This is probably a breaking change, so it might be a good idea to increment the public API before/after this is merged and bundle some more breaking changes in.

Follow up changes:

• (Make rbx_reflection API less public #35) Make rbx_reflection API less public (getters instead of pub fields)
  • Hide Cow types that are leaking right now
  • Change names to include words like Reflection and Descriptor
• (Configurable serializer/deserializer #34) Serializer/deserializer options like turning off reflection guidance (raw mode?)
• (RbxValue try_convert/try_convert_ref API #36) Nail down RbxValue::try_convert/try_convert_ref API, document it, since it changes rbx_dom_weak
• (Expand reflection test suite #37) Expand test suite where reflection-guided serialization might fall down

[LPGhatguy]

Going to push off TODOs into future changes and push to merge this!