This commit adds the code necessary to read and write the `meta` table,
including its codegen definition, manual extras to handle the variable
length data fields, and plumbing to surface it within `read-fonts` and
`write-fonts`.

Where possible, this commit borrows liberally from the implementation of
other tables, especially `name` (for its handling of variable length
data).

This is an early implementation, and as such requires manual and
automated testing. Additionally, there is an implementation of the
FontWrite trait for &[u8] that may be redundant.

This adds types and the required impls that can represent the expected
contents of the 'meta' table.

This has to be handwritten, because the 'meta' table contains different
payloads depending on the included tag for each record.

I think having these as real types will provide a significantly better
API though, and avoid us writing invalid data.

The actual implementations for these types are very spartan; eventually
I imagine having parsing & constructing code for the ScriptLangTag
struct that ensures it conforms to the spec.