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.