Make Read and Show derivation for Route _ optional

[parsonsmatt]

We've identified that Read is particularly slow to derive - twice as slow as Eq or Show. A 1000 constructor sum type with a single Int parameter in each constructor takes about 2.5s on my laptop to compile, while Eq is a mere 0.8s and no derivation is 0.2s (for reference).

If I remove the Read derivation in the mkYesod functions, all tests in yesod-core pass fine, and all tests pass in the work repository with this change too.

#1203 is related - by dropping the Read derivation, we no longer need Read in elements in the path pieces.

[snoyberg]

No objection from me, seems like a reasonable change. How were you thinking of indicating this? The route syntax may be a good place

[parsonsmatt]

I was probably going to port the code from persistent that allows you to specify deriving stuff by default for keys and models, which would introduce new named functions mkYesod{,Dispatch,etc}With that accept that record as an argument

[snoyberg]

Cool, sounds like a good plan.

[dten]

@parsonsmatt did you give this a try? We would also like to not require Read