User-definable functions for builtin Haskell syntax

[jespercockx]

It would be nice if we could make some of the functions that are currently defined in the Prelude (such as if_then_else_) user-definable instead, and perhaps even allow multiple Agda functions that compile to the same Haskell syntax. Here is a list of some functions for which this could be done:

• [x] Tuple constructors _,_, _,_,_, ...
• [ ] if_then_else_
• [ ] case_of_
• [ ] Integer and string literals
• [ ] enumFromThenTo
• [ ] ...

[jespercockx]

See also #74 for some motivation

[jespercockx]

The types of if_then_else_ and case_of_ are now more general so there's less need for having multiple versions of them, and for literals and ranges the need to have multiple versions of them is a bit dubious too. So since we have the most important part done, I will close this now so it can be added to the 1.3 milestone.