Extend KeyPath to allow CartesianIndex keys

[jondeuce]

I ran into this issue while trying to use fmap_with_path while implementing an optimizer whose state is represented as a Matrix{NamedTuple}:

julia> fmap_with_path((k, x) -> (@show(k); x), [(a=1,) (b=2,)]);
ERROR: MethodError: no method matching KeyPath(::KeyPath{Tuple{}}, ::CartesianIndex{2})
The type `KeyPath` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  KeyPath(::Union{AbstractString, Integer, Symbol, KeyPath}...)
   @ Functors ~/.julia/dev/Functors/src/keypath.jl:60
  KeyPath(::T) where T<:Tuple
   @ Functors ~/.julia/dev/Functors/src/keypath.jl:55

Stacktrace:
  [1] (::Functors.var"#30#31"{KeyPath{Tuple{}}})(c::CartesianIndex{2})
    @ Functors ~/.julia/dev/Functors/src/walks.jl:80
  [2] iterate
    @ ./generator.jl:48 [inlined]
  [3] _collect
    @ ./array.jl:800 [inlined]
  [4] collect_similar
    @ ./array.jl:709 [inlined]
  [5] map
    @ ./abstractarray.jl:3371 [inlined]
  [6] _map
    @ ~/.julia/dev/Functors/src/walks.jl:3 [inlined]
  [7] DefaultWalkWithPath
    @ ~/.julia/dev/Functors/src/walks.jl:80 [inlined]
  [8] ExcludeWalkWithKeyPath
    @ ~/.julia/dev/Functors/src/walks.jl:135 [inlined]
  [9] (::Functors.CachedWalkWithPath{Functors.ExcludeWalkWithKeyPath{…}, Functors.NoKeyword, Functors.WalkCache{…}})(::Function, ::KeyPath{Tuple{}}, ::Matrix{NamedTuple{…} where names})
    @ Functors ~/.julia/dev/Functors/src/walks.jl:199
 [10] execute(walk::Functors.CachedWalkWithPath{Functors.ExcludeWalkWithKeyPath{…}, Functors.NoKeyword, Functors.WalkCache{…}}, x::KeyPath{Tuple{}}, ys::Matrix{NamedTuple{…} where names})
    @ Functors ~/.julia/dev/Functors/src/walks.jl:55
 [11] fmap_with_path(::Function, ::Matrix{NamedTuple{names, Tuple{Int64}} where names}; exclude::Function, walk::Functors.DefaultWalkWithPath, cache::IdDict{Any, Any}, prune::Functors.NoKeyword)
    @ Functors ~/.julia/dev/Functors/src/maps.jl:23
 [12] fmap_with_path(::Function, ::Matrix{NamedTuple{names, Tuple{Int64}} where names})
    @ Functors ~/.julia/dev/Functors/src/maps.jl:14
 [13] top-level scope
    @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

I can of course change the state representation, but I think this is a rather harmless extension to KeyPath. After this patch:

julia> fmap_with_path((k, x) -> (@show(k); x), [(a=1,) (b=2,)]);
k = KeyPath(CartesianIndex(1, 1), :a)
k = KeyPath(CartesianIndex(1, 2), :b)

Version info:

(jl_i3jhdk) pkg> st
Status `/tmp/jl_i3jhdk/Project.toml`
  [d9f16b24] Functors v0.5.1

julia> versioninfo()
Julia Version 1.11.1
Commit 8f5b7ca12ad (2024-10-16 10:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × AMD Ryzen 9 3950X 16-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 32 default, 0 interactive, 16 GC (on 32 virtual cores)