Remove Yi.Prelude

Move utility functions from Yi.Prelude into Yi.Utils
Merge pull request #490 from fread2281/no-prelude

Remove Yi.Prelude
Reformat module
Make justOneSep behave more like just-one-space

Previously, with text like (where | denotes the cursor)
"hello    |world" we could not use justOneSep to remove the extra space.
Now we can.

This also inserts a single spaces where there aren't any separators
already, which is consistent with the emacs function.