Add Function.Extra!?

Not sure if there is much appetite for this, but a Core-ish type is the function type. There are a bunch of helper functions to do with functions:

- `uncurry : (a -> b -> c) -> (a, b) -> c`
- `curry : ((a, b) -> c) -> a -> b -> c`
- all sorts of function combinators: `a -> b -> b`, `(a -> c) -> (b -> c -> d) -> a -> b -> d`, yada yada
- `toFixpoint : (a -> a) -> a -> a` and perhaps `toFixpointWithLimit : Int -> (a -> a) -> a -> Result a a`
- `while : (a -> Bool) -> (a -> a) -> a -> a` and `doWhile : (a -> a) -> (a -> Bool) -> a -> a`
- `nTimes : Int -> (a -> a) -> a -> a`

etc...

Thoughts? Opinions?

The issue proposes a new Function.Extra module for Elm with currying, combinators, fixpoint, looping, and repetition helpers, but names no files or tests. First review the existing core-extra modules and Elm core conventions, then narrow the API and define tests that establish the intended behavior before implementation.

Written by the indexing model from the issue text.