Use FillArrays.simplify_broadcasted for Zeros broadcasts

[jishnub]

Depends on JuliaArrays/FillArrays.jl#444 (FillArrays 1.19). CI will not resolve until that is merged and released — the compat bound here is raised to FillArrays = "1.19".

Why

The Zeros rules in generic/broadcast.jl called FillArrays._broadcasted_zeros, an internal imported at the top of BandedMatrices.jl, and duplicated FillArrays' own list of which operations a Zeros absorbs (*, / one way; *, \ the other). If FillArrays gains or changes a rule, this list silently falls out of step.

What changes

FillArrays.simplify_broadcasted(style, op, args...) returns the simplified array where a rule applies, and otherwise a Broadcasted carrying the style we hand it — so anything it cannot simplify comes straight back for BandedStyle to handle as before.

broadcasted(S::BandedStyle, op, a::AbstractZeros, b::AbstractArray) = FillArrays.simplify_broadcasted(S, op, a, b)
broadcasted(S::BandedStyle, op, a::AbstractArray, b::AbstractZeros) = FillArrays.simplify_broadcasted(S, op, a, b)
broadcasted(S::BandedStyle, op, a::AbstractZeros, b::AbstractZeros) = FillArrays.simplify_broadcasted(S, op, a, b)

No operations are enumerated, and _broadcasted_zeros is dropped from the import list.

The Ones rules are deliberately left as they were: they preserve bandedness via _copy_oftype, which is our semantics rather than FillArrays', and routing them through simplify_broadcasted would lose that.

One incidental fix: the Zeros methods keyed on the concrete Zeros, so AbstractZeros subtypes were missed. They now key on AbstractZeros. The third method disambiguates the first two when both arguments are zeros.

Test suite passes against FillArrays#444.

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