Incorporate block-sizes into axes

[dlfivefifty]

I've always felt the design for block sizes is not very elegant, and the termonology is completely unintuitive (even after acting as primary maintainer for this package for several years). I believe a better design would move this to be part of the axes of an array. That is, we would have the following behaviour:

bs = [1,5,3] # block sizes
n = sum(bs) # total length of vector
x = BlockVector(randn(n), bs) # returns a Block vector with block-sizes [1,5,3]
a = axes(x,1) 
a === BlockSlice(Base.OneTo(n), bs) # BlockSlice plays same role as `Slice`
length(a[Block(2)]) === 5 # returns the size of the second block
a[Block(2)][3] === 4 # returns the "global" index into `x` of the 3rd entry of the second block. 
# This replaces blockindex2global:
x[a[Block(2)][3]] === x[4]