GitLab

Refactor absolute error computation (step 1)

See merge request !79

The following has changed:
- absolute error computation previously present in ErrorFunctions
  is now split into range computation and error computation.
- it does not annotate the tree anymore, the information is passed via maps in the context
- error computation supports mixed-precision (without a front-end)

What is probably broken or needs fixing:
- fixed-point code generation
- RangePhase and AbsErrorPhases cleanup and deduplication

Rework evaluation semantics to not be arguing about precondition, make this explicit in the theorem, that we assume it. Admitted proofs that are obvious