Add haskey for Vector

[dpsanders]

In order to write generic code that can use Vector and Dict interchangeably, it would be very useful to add haskey(v, i) for a Vector v, to check if v[i] exists.

Since there is already keys(v) for a Vector, this seems consistent.

Daniel Berge suggested on Slack to use i in keys(v), which seems to work, but also seems like an unnecessary workaround.

[yuyichao]

#18843

[yuyichao]

#25105

[vtjnash]

Should we rename isassigned to haskey?

[StefanKarpinski]

They don't answer the same question really. Vector{Any}(undef, 3) has keys 1:3 but none of those slots is assigned.

[vtjnash]

Vector doesn't have haskey though, it is a Dict function, and it tests whether those keys are valid