Users need advice on where to find givens

[propensive]

@implicitNotFound messages should be provided for every typeclass.

[propensive]

A quick grep revealed that there are about 100 typeclasses which need explanation. That explanation should generally include:

• what the typeclass is used for, and why you might need to make a choice
• where to import one from and what a sensible default might be, or
• how to construct one

[propensive]

We can start by storing them all in a package whose name is derived from the name of the typeclass. For example Identifiable instances should be kept in a the identifiables package—not enumIdentification.