Squash implementation missing

[josephcbradley]

Some squash implementations are missing:

 Warning: squash(::AbstractGraph) is deprecated in favor of methods that specialize on the graph type.
│   caller = macro expansion at core.jl:112 [inlined]
└ @ Core ~/work/Graphs.jl/Graphs.jl/test/core.jl:112

Just needs the specialisations function checking for which types are missing. Happy to take.

[Krastanov]

Thanks for reporting! If you have a chance to address it, please submit a PR -- you know already, but we are pretty short-staffed.