Unary broadcasting `identity.(a)` and `ComplexF64.(a)` don't preserve `Ones` and `Zeros`

[mtfishman]

For example:

julia> identity.(Zeros(2))
2-element Fill{Float64}, with entries equal to 0.0

julia> identity.(Ones(2))
2-element Fill{Float64}, with entries equal to 1.0

julia> ComplexF64.(Zeros(2))
2-element Fill{ComplexF64}, with entries equal to 0.0 + 0.0im

julia> ComplexF64.(Ones(2))
2-element Fill{ComplexF64}, with entries equal to 1.0 + 0.0im

julia> complex.(Zeros(2))
2-element Fill{ComplexF64}, with entries equal to 0.0 + 0.0im

julia> complex.(Ones(2))
2-element Fill{ComplexF64}, with entries equal to 1.0 + 0.0im

julia> pkgversion(FillArrays)
v"1.13.0"

julia> versioninfo()
Julia Version 1.11.6
Commit 9615af0f269 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)
Environment:
  JULIA_EDITOR = code

[jishnub]

This would be fixed by #385, but we may specialize some cases until that's merged.

[dlfivefifty]

I think specializing only makes sense if there is some need downstream, otherwise just wait til the general fix

[mtfishman]

I think specializing only makes sense if there is some need downstream, otherwise just wait til the general fix

I have some use cases where this would be helpful but I can work around it for the time being.

[dlfivefifty]

#440 fixes the zero cases. The one cases are harder. because there's no LinearAlgebra.fonepreserving