Add (optional) serde support to write-fonts

I would like to be able to write tests for write-fonts that involve attempting to serialize specific real-world failures, and the best way I can think of doing this is to add optional serde support; this way I can save some specific table to disk and then load and compile it in tests.

This will require also adding (optional) serde support to font-types; see #311.

Start by inspecting the write-fonts and font-types crates and their dependency configuration to understand how optional features are defined. Add optional serde support across both crates so tests can save and reload tables, then verify that the real-world failure cases can be serialized and compiled in tests.

Written by the indexing model from the issue text.