Broadcasting with FillArray allocates and is much slower

[rmsrosa]

I found an undesirable allocation when drawing from a MvNormal(I(n)) which boils down to broadcasting with FillArrays:

julia> u = rand(2); v = Zeros(2); w = collect(v);

julia> f(u, v) = ( u .+= v)
f (generic function with 1 method)

julia> @btime f($u, $v)
  47.824 ns (1 allocation: 80 bytes)
2-element Vector{Float64}:
 0.17323378731244632
 0.4293927595304291

julia> @btime f($u, $w)
  5.292 ns (0 allocations: 0 bytes)
2-element Vector{Float64}:
 0.17323378731244632
 0.4293927595304291

Might be somewhat related to #188 and #58

[jishnub]

This isn't related to those issues, it's simply a badly implemented broadcasted method that materializes the result. Should be an easy fix.