support StaticArrays

It would be nice if the various `rand(...)` functions supported returning an `SMatrix`, for example if `::Val{n}` is passed instead of `n::Integer`.

This improves performance and reduces memory allocations in cases where lots of small matrices need to be sampled. e.g. see [this example from discourse](https://discourse.julialang.org/t/code-speed-decreases-when-loop-number-increases/78920) which generates lots of 2x2 and 4x4 Haar-distributed unitary matrices.

Start by locating the various rand(...) functions in RandomMatrices.jl and review how they currently accept integer dimensions. Check how an SMatrix and a Val{n} argument could fit the existing APIs, then verify that small Haar-distributed unitary matrices can be returned without the stated allocation and performance costs.

Written by the indexing model from the issue text.