Add FillArrays extension

[devmotion]

This PR adds FillArrays as a weak dependency on Julia >= 1.9 and as a proper dependency on Julia < 1.9 (the latter might not be completely desirable, see #182 (comment), and alternatives would be to use Requires or reduce features on older Julia versions).

Currently, the only method in the extension is a definition of AbstractPDMat for Diagonal matrices with an FillArray as diagonal. This is motivated by the constructors of MvNormal in Distributions in https://github.com/JuliaStats/Distributions.jl/blob/87aebc29b2b9608801b70aae09fbc1d2dad56e3f/src/multivariate/mvnormal.jl#L201-L210: With the definition in this PR they could be simplified to

MvNormal(μ::AbstractVector{<:Real}, Σ::AbstractMatrix{<:Real}) = MvNormal(μ, AbstractPDMat(Σ))
MvNormal(μ::AbstractVector{<:Real}, Σ::UniformScaling{<:Real}) =
    MvNormal(μ, ScalMat(length(μ), Σ.λ))

This pattern is more generally useful, I imagine: Downstream packages that deal with pd matrices could be encouraged to use AbstractPDMat(...) for converting user-provided matrices to an optimized representation. The PR here adds such optimizations also for Diagonal with FillArrays as diagonal.

This is an alternative to #184 that keeps support for Julia 1.0. The issues with Pkg.test (incompatibilities of test dependencies with already installed package dependencies) can be avoided by enforcing the use of FillArrays 0.10 on Julia 1.0. Julia 1.0 is also supported by FillArrays 0.11 and 0.12 and hence I added them to the compat entry as well; however, the test dependencies BandedMatrices and ArrayLayouts can't be installed together with these more recent versions on Julia 1.0, hence I explicitly downgraded FillArrays to 0.10 in CI on Julia 1.0.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
ext/PDMatsFillArraysExt.jl	100.00% <100.00%> (ø)
src/PDMats.jl	100.00% <ø> (ø)

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[devmotion]

Closing since FillArrays would increase loading times significantly: #182 (comment)