Version 0.5.1

• Catch more errors in client code:
  • Validate arity of callable value in letrec (both versions)
  • Validate callability and arity of body in all let constructs
  • In env, require name to be an identifier even when subscripting
• Add flag to enable_fasttco() to be able to switch it back off (during the same run of the Python interpreter)
• Internal enhancement: add unit tests for env