Feature request: ability to specify method table in methodinstance

[MasonProtter]

It'd be useful if methodinstance could work with any method table, not just the global default one.

The version that takes a worldage is easy, it just needs to become

function methodinstance(ft::Type, tt::Type, world::Integer; methodtable=nothing)
    sig = typed_signature(ft, tt)

    match, _ = CC._findsup(sig, methodtable, world)
    match === nothing && throw(MethodError(ft, tt, world))

    mi = CC.specialize_method(match)

    return mi::MethodInstance
end

but I tried to do it in the generated function version and I couldn't figure out how to get it to work properly without segfaulting, so any hints or help on making that work would be much appreciated.

[maleadt]

We're likely to change the methodinstance implementation to re-use jl_lookup_generic, see #530, so a methodtable would need to be added there.