Replace getblock(A, k) with view(A, Block(k))

[dlfivefifty]

We have two different versions of getting a block without copying: getblock(A, k::Int...) and view(A, ::Block...). I propose deprecating getblock and having the canonical version the latter. Note this would mean view does not (always) return a SubArray, but this is fine:

julia> view(2:5, 2:3) # returns a range as type is immutable
3:4

Note this might break some code: I recall for PseudoBlockArray purposefully adding BlockSlice to retain the information about which block a range was coming from. I don't exactly recall why this was necessary, but I suspect it was for BlockBandedMatrices.jl where we needed to recognise that view(A, Block(K,J)) was banded. That said, with the proposed redesign this will return a BandedMatrix which is even nicer. And probably when I designed the code I thought it wasn't allowed to return something other than a SubArray, but my view has changed as of an hour ago.

Cf JuliaLang/julia#38536 JuliaLang/julia#38536