allow printing some TOML dictionaries inline by marking them with an IdSet

[KristofferC]

This was one way I came up with to "mark" what dictionaries one wants to print inline. I am not sure IdSet is even considered public but it seemed the most reasonable for this? I want to use this in Pkg to write some TOML files more terse.

cc @StefanKarpinski

[KristofferC]

This is not correct:

d = Dict("deps" => Dict(
        "LocalPkg" => "fcf55292-0d03-4e8a-9e0b-701580031fc3", 
        "Example" => "7876af07-990d-54b4-ab0e-23690620f79a"),
   "sources" => Dict(
        "LocalPkg" => Dict("path" => "LocalPkg"),
        "Example" => Dict("url" => "https://github.com/JuliaLang/Example.jl")))

inline_tables = Base.IdSet{Dict}()
if haskey(d, "sources")
    for source in values(d["sources"])
        push!(inline_tables, source)
    end
end

TOML.print(d; sorted=true, inline_tables)

prints (loses the [sources]):

[deps]
Example = "7876af07-990d-54b4-ab0e-23690620f79a"
LocalPkg = "fcf55292-0d03-4e8a-9e0b-701580031fc3"
Example = {url = "https://github.com/JuliaLang/Example.jl"}
LocalPkg = {path = "LocalPkg"}

[vtjnash]

LGTM