Configurable serializer/deserializer

[LPGhatguy]

It should be possible to configure both the serialization and deserialization methods in rbx_xml.

The only big option I think we need right now is the ability to turn off reflection-driven serialization.

This is a good opportunity to break the API to pick better names. I like the idea of picking names inspired by serde_json's API (but with a better name for the options struct):

• rbx_xml::from_str(tree: &mut RbxTree, id: RbxId, source: &str, options: DeOptions)
• rbx_xml::to_writer(output: &mut W, tree: &RbxTree, ids: &[RbxId], options: SeOptions)

[LPGhatguy]

Done for 0.7.0!