Enum length-check fixes + Enum optional field support (#217)

* Enum length-check fixes + Enum optional field support

Fixes #175

Now properly checks all lengths for all variants to ensure that
overlapping types parse the correct variant instead of prematurely
thinking it's a subset of one.

Also fixes having CBORReadLen contributions from previous variants that
tried to parse from contributing to later variant parses (possibly
causing issues if it meant it already hit the limit).

Includes support for optional fields within enums that get inlined.

Tests for both cases.

* preserve-encodings tests for overlapping_inlined + enum_opt_embed_fields