Rework block generator

This is meant to redo some of the block generator stuff so that it stores all the blocks in a huge linear array. It's still a work in progress, but I wanted to post it because I'm not sure I'm doing the scans and setup properly.

It's just for P2P at the moment and it basically does the following (in P2PMatrixBlockGenerator.__call__):

• Creates a little kernel that computes a cumsum of block sizes.
• Creates a little kernel that computes a set of (rowindices, colindices) that are meant to map from the linear index to the row and column in the full matrix.
• Uses the row and column indices to fill in the results array with all the block matrix elements. This is a bit nicer now because we can split a big iname and scale better (hopefully).

What I was wondering:

• Is there a cleaner way to construct the cumulative (rowindices, colindices)? I couldn't think of any..
• Should I merge the first two loopy kernels? I separated them because the first one basically computes the size of rowindices and colindices and I couldn't think of another way to tell loopy what size they're supposed to be.