define keys for Base.Generator

[KlausC]

IMO findmin(sin(x) for x in 0:10) should return same as findmin([sin(x) for x in 0:10]),
without allocating. Currently the first call throws a MethodError.

This PR inserts key(::Base.Generator) in order to achieve that.
As a consequence also pairs(::Generator), findmin(::Genearator), findmax(::Generator) are working as expected.

julia> @btime findmin(sin(x) for x = 1:10^6)
  21.901 ms (0 allocations: 0 bytes)
(-0.9999999999848337, 52174)

julia> @btime findmin([sin(x) for x = 1:10^6])
  25.945 ms (2 allocations: 7.63 MiB)
(-0.9999999999848337, 52174)

[fredrikekre]

See #34678 (which actually has another definition of keys(::Generator)).

[KlausC]

I retire this one in favour of #34678 . Sorry for the inconvenience.