Support for types like Int with U <: UnitExpr

[erikerlandson]

This might be a very powerful feature, although it comes with some drawbacks and problems to solve:

1. the with U part isn't closed under various numeric operations
2. easy to drop the with U part by assigning to non-qualified numeric values
3. have to figure out rules and support for operations with differing types
4. e.g. what is result type of Int with U <+> Float with U
5. Numeric[N] doesn't seem to support construction of new N from arbitrary other values

[erikerlandson]

@non, do you have any advice on whether spire can help with problems 3,4,5 above?
Currently I'm just sort of punting by doing everything in Double space, which isn't terrible but I feel like it could be better and standard Scala Numeric doesn't quite get me there.

[erikerlandson]

I have decided to not direcly use with, instead it is safer to use a type alias WithUnit and/or %# such that Int WithUnit Meter is equivalent to Quantity[Int, Meter]. This also makes it behave more like require
I'm closing this issue.