Generic interface for constructing `AbstractBlockArray`s: `blocked(a::AbstractArray, blocklengths...)`

[mtfishman]

This is a proposal for a new generic function for constructing block arrays, blocked(a::AbstractArray, blocklengths...), which takes an AbstractArray a and block lengths blocklengths, one for each dimension, and outputs the closest reasonable AbstractBlockArray. It would be a generalization of the corresponding BlockArray and PseudoBlockArray constructors, and could make a PseudoBlockArray as a fallback.

This could be used for making BlockedUnitRanges with blocked(1:7, [4, 3]) == blockedrange([4, 3]) and BlockedOneTo (being developed in #348) with blocked(Base.OneTo(7), [4, 3]). It may also call for defining BlockedUnitRange(1:7, [4, 3]) and BlockedOneTo(Base.OneTo(7), [4, 3]), again analogous to the BlockArray and PseudoBlockArray constructors, but I'm not sure if that is needed.

blocked(a::AbstractArray, blocklengths...) would be very similar to a[blockedrange.(blocklengths)...], however, so maybe it isn't needed and that is the preferred syntax for this operation. However, blocked could make a view while a[blockedrange.(blocklengths)...] copies.

See #347 (comment) for a related discussion.