Add Array{T,N}(nothing/missing, dims) convenience constructors

[nalimilan]

These are shorthands for fill!(Array{T,N}(uninitialized, dims), nothing/missing). In the future they could be optimized for isbits Union types in which nothing/missing is the first type of the Union, since such arrays are already filled with nothing/missing.

This implements solution 3. from #24939, which got broad support.

I haven't found a way to attach the new docstrings to the new methods rather than to the general docstring for the function/type, given that methods are generated using @eval with a loop. Anyway the docstrings describe both the nothing and the missing method, so that sounds unavoidable to limit duplication.

[nalimilan]

The new definitions cause weird redefinition warnings during bootstrap:

WARNING: Method definition (::Type{Array{T, N}})(Void, Integer...) where {T, N} in module Inference at array.jl:398 overwritten in module Base at array.jl:398.
WARNING: Method definition (::Type{Array{T, N}})(Void, Tuple{Vararg{Integer, N}}) where {T, N} in module Inference at array.jl:400 overwritten in module Base at array.jl:400.
WARNING: Method definition (::Type{Array{T, N} where N})(Void, Integer...) where {T, N} in module Inference at array.jl:403 overwritten in module Base at array.jl:403.
WARNING: Method definition (::Type{Array{T, N} where N})(Void, Tuple{Vararg{Integer, N}}) where {T, N} in module Inference at array.jl:405 overwritten in module Base at array.jl:405.

For some reason, they do not happen with the ::Missing methods.

[fredrikekre]

Where these not included in the Array entry on the line above?

[nalimilan]

No, I got an error:

 !! No docs found for 'Core.Array(::Any, ::Any)'. [src/stdlib/arrays.md]

I thought there was a way to include all methods, but this one doesn't work at least.

[fredrikekre]

Right, I meant that you should remove Core.Array(::Any) and Core.Array(::Any, ::Any) and keep Core.Array. I think that Core.Array should catch everything associated with Array, but perhaps there was some reason for including them all earlier, perhaps to place the type docstring for Array at the top?

[nalimilan]

No, that only includes the type, not methods.

[fredrikekre]

Ok, good to know!

[nalimilan]

I've found a way to avoid the "method redefined" warnings by defining the code in sysimg.jl together with existing Array constructors. I'll merge once CI passes unless somebody objects.

[fredrikekre]

LGTM, lets give @Sacha0 the opportunity to review since he is the one engineering these type of array constructors :)

[nalimilan]

AppVeyor failure on 64-bit is the infamous Bunch-Kaufman bug.