type-valued float and real functions

[stevengj]

Since we often treat StaticArrays almost like scalars, it would be nice to have float and real functions that act on the types. i.e.

float(::Type{SArray{S,T,N,L}}) where {S,T,N,L} = SArray{S,float(T),N,L}
real(::Type{SArray{S,T,N,L}}) where {S,T,N,L} = SArray{S,real(T),N,L}

analogous to e.g. float(Int) === Float64 and real(ComplexF32) == Float32 in Base.

Basically float(T) === typeof(float(zero(T))), and similarly for real.