Various fixes to constants

Frederik Hennigrequested to merge
fhennig/fix_constants into backend-rework

Various fixes to constants:

  • Make PsConstant immutable
  • Add interpret_as/reinterpret_as to apply data types to constants
  • Add docstrings to PsConstant
  • Add out-of-bounds checking to constant creation
  • Add test cases
  • Remove long-obsolete and completely commented-out test files
Edited by Frederik Hennig

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