feature: make PtrArray{T}(undef, dims...) an alias for malloc(T, dims...)

[nsajko]

Other array types, like Memory and Array, support being constructed with undef.

This also means the malloc function is redundant and could be deleted if a breaking release ever happens.

[LilithHafner]

Thanks for offering this! It is actually an intentional omission: I don't want to hide a leaky malloc call behind an innocuous front-end like PtrArray{Int}(undef, 5). As you demonstrate in the tests you wrote, this is a memory leak foot-gun because people don't expect undef initialization to leak memory (whereas I would expect folks to be vigilant for memory leaks when using malloc).