This adds support for parsing/processing `tag` and `tag_field` options
for structs. This is only the first step for supporting tagged unions,
they're not currently used anywhere outside of the StructMeta
constructor.

Adds support for JSON & MessagePack encoders to encode tagged structs.

This implements the first half of decoding Struct unions (validating and
building the lookup table when creating a decoder). Decoding these types
is still not supported.

Supports decoding JSON & MessagePack into Unions containing multiple
tagged structs. Still needs tests.