Draft: Avoid recursive block definitions.

In algorithms that require recursive calls on blocks (e.g. as required by tlapack), recursive block class definitions can lead to infinite compile loops, causing the compiler to crash. Since blocks are contiguous sections of an underlying expression, we should easily be able to avoid this by referring to the original underlying expression when taking a block of a block. This should also end up being more efficient.

This came up in discussions with Weslley Pereira.