begin to document all kernel function in elm/core

[harrysarson]

Here is a start to address #50.

I think the best way to contribute to this is to make "review suggestions" in this pull request which can then be merged in. See #57 (comment) as an example.

As a good start the table should list all kernel functions in elm/core, then adding notes and the places where is function is used after. :)

@Janiczek I think this is a good candidate for help wanted.

[harrysarson]

@Janiczek not only do we have to contend with kernel functions but also kernel global mutable state. I believe that compiler generated code interacts with these variables too!

[Janiczek]

Oooh. This will be interesting. Thanks for finding this out!