IdentityMatrix.jl: efficient methods

There are a lot of possible efficiency improvements for operations involving identity matrices. This package is intended to get several improved (or specialized) methods into circulation. This is not intended to be their final form and home.

Some examples are: faster iterate for FillArrays.Eye; Faster construction of a dense identity matrix than Matrix{T}(I, n, n); Faster kron. There are many more. A partial list can be found in the README.