Made 'arr' work as 'id' for any single category.
Introduced well-pointed categories.

These are now yet another prerequisite for 'Arrow'; they should allow many of the 'Monad' etc. utilities to be written in a better way, and allow proxies to feature constant values ('point's).
Added affine mappings to the vectorspace example.

Unlike linear mappings, these form a cartesian closed category, which is a bit more interesting for the example purpose.
Updated the rest of the library, and examples, for the inclusion of 'WellPointed'.
Small documentation cleanups.
Added '(<<)' to monad module.

It doesn't have its Hask-specific pendant in Control.Monad, but this function arises very naturally (indeed, more naturally than '(>>)'.