Convert typedef hash to new hash table

This converts the typedef hash to use the new hash table.

This patch found a latent bug in the typedef code.  Previously, the
hash function looked at the type name, but the hash equality function
used types_equal -- but that strips typedefs, meaning that equality of
types did not imply equality of hashes.  This patch fixes the problem
and updates the relevant test.

Change-Id: I0d10236b01e74bac79621244a1c0c56f90d65594
Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>