Generate functions from enums to numbers and vice versa

For an enumeration type T, we can create a function T_of_num and num_of_T which convert from the enum to and from a numeric type. The numeric type is range(0, n) where n is the number of constructors in the enum minus one. This makes sure the conversion is type safe, but maybe this is too much of a hassle.

It would be possible to automatically overload all these functions into generic to_enum and from_enum as in Haskell's Enum typeclass, but we don't do this yet.

Currently these functions affect a few lem test cases, but I think that is only because they are tested without any prelude functions and pattern rewrites require a few functions to be defined

What is really broken is if one tries to generate these functions like

enum x = A | B | C

function f A = 0
function f B = 1
function f C = 2

the rewriter really doesn't like function clauses like this, and it seems really hard to fix properly (I tried and gave up), this is a shame as the generation code is much more succinct with definitions like above

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