Remove the def-id from type parameters. Having this def-id was bad for several reasons:

Remove the def-id from type parameters. Having this def-id was bad for several reasons:

1. Produced more unique types than is necessary. This increases memory consumption.
2. Linking the type parameter to its definition *seems* like a good idea, but it
   encourages reliance on the bounds listing.
3. It made pretty-printing harder and in particular was causing bad error messages
   when errors occurred before the `TypeParameterDef` entries were fully stored.