Write a TH linter

[goldfirere]

It is very easy to get dimension and unit declarations wrong, leading to terrible error messages. This feature request is to add a "lint" function, written in Template Haskell, to check unit and dimension definitions for sanity and report decent error messages.

[goldfirere]

This function should be called after the definitions themselves and will use reification to check the definitions -- the def'ns won't be in a TH quote.