Add flattening, unindexed, unlabeled, dynamic, transparent support

[KodrAus]

This PR is a big feature enhancement to the derive macros. It adds the following new attributes:

• #[sval(unlabeled_values)] and #[sval(unindexed_values)]: streams a struct as a tuple if unlabeled, as a record if unindexed, and as a sequence if both unlabeled and unindexed.
• #[sval(dynamic)]: makes enums untagged.
• #[sval(transparent)]: makes a newtype stream as its single field.
• #[sval(flatten)]: flattens enum variants, the key-values of maps, values of sequences, and fields of records and tuples onto their parent value.

I've done some major refactoring of the derive macros to support all this new stuff; it's basically rewritten. We still need to do proper error reporting, but are making incremental steps towards something more broadly useful.