Rewrite TR class form to preserve abstractions

This uses a technique discovered by Ryan and Dan that
allows the typed class macro to function without invasively
local-expanding the entire class macro (using its private
context information).

Instead, it expands into many helper macros inside the
normal class body and communicates among them using
`syntax-local-value` and compile-time state within the
class body.

This rewrite didn't save that many lines, but it did
reduce the amount of magic that's used.