Feature request: ability to specify method table in `methodinstance`

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
```julia
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.

Start by locating the existing `methodinstance` overloads and compare the worldage version with the generated-function version. Read the uses of `typed_signature`, `CC._findsup`, and `CC.specialize_method`; the work is complete when a supplied method table works for both forms without segfaulting and returns the expected `MethodInstance`.

Written by the indexing model from the issue text.