Change Read and Show instances to use fromList

[konsumlamm]

Fixes #117.

This is a breaking change, as it changes the behaviour of the Read and Show instances and adds an Ord constraint to the Read instances.

[konsumlamm]

@treeowl what do you think?

[treeowl]

I prefer for Read instances to be permissive. The fromList should be optional.

[konsumlamm]

I prefer for Read instances to be permissive. The fromList should be optional.

You mean it should accept fromAscList/fromDescList or fromList?

[treeowl]

What I meant is that it should also accept bare list syntax, a la OverloadedLists.

[konsumlamm]

There are currently no IsList instances, but I'm not opposed to adding them (containers and unordered-containersalso have them). In my opinion, bare list syntax should only be accepted if there are IsList instances.

Are you okay with removing support for fromAscList/fromDescList in Read/Show instances?

[treeowl]

fromAscList and fromDescList seem pretty interesting to support properly, but probably not very useful. So yes, get rid of them.

(Proper behavior would be to produce parse failure if they're not actually ascending/descending, but Read doesn't have the sort of error handling mechanisms needed to make that helpful.)