TOML: Improve type-stability

[topolarity]

Dependent on #55017. This changes the output of the TOML parser to provide specialized Vector{T} less aggressively.

Specifically, combinatorially expensive types like Vector{Vector{Float64}} or Vector{Union{Float64,Int64}} are instead returned as Vector{Any}, while vectors of homogeneous leaf types (e.g. Vector{Float64}) are still supported as before.

This change makes the TOML parser fully type-stable.

[KristofferC]

Probably good to parameterize here then:

julia/base/loading.jl

Lines 267 to 270 in ce4f090

	struct TOMLCache
	p::TOML.Parser
	d::Dict{String, CachedTOMLDict}
	end

to not pessimize the uses of it.

However, Pkg.jl also uses this TOMLCache and wants to have Dates enabled while the loading code does not. So somehow those uses have to be "split up" I guess.

[topolarity]

However, Pkg.jl also uses this TOMLCache and wants to have Dates enabled while the loading code does not. So somehow those uses have to be "split up" I guess.

Is it coherent to have them share the cache, if one returns different objects than the other? That sounds problematic..

[KristofferC]

Yes, Pkg should probably have its own completely separate cache and if some project files are parsed twice (in Base and Pkg) then I guess that is whatever.

[KristofferC]

This cache https://github.com/JuliaLang/Pkg.jl/blob/8c996799b0ae3d6cccf8a5f25744deb6640adb9e/src/Registry/registry_instance.jl#L18-L21 should probably thus be lifted up to the Pkg module and be used by all parsed_toml functions in Pkg.

[topolarity]

Actually it looks like that cache is already separate, along with the one in Types.jl

[topolarity]

OK, I've parameterized TOMLCache now and I think this should be ready to go again.

#55017 should be reviewed first, since this just adds one commit on top