Drop old serde support and move serde 1.0+ support in-crate

[brson]

Having serde support in its own crate is not the ideal pattern for serde. It would be better for the serde feature to be for modern serde. Changing this though requires a major version bump.

@dtolnay has a plan for getting serde 1.0 support in-crate using a slightly uglier pattern in the meantime.

[dtolnay]

Moving Serde support in-crate is tracked in #304 so this is just tracking dropping the old Serdes.

[nox]

This was done in master.