share the commit of FRI blocks using RC

This MR changes the fri::Block.commit type. It now uses an RC to share the memory through all the Blocks and avoid memory copies of something that can be huuuuuuuuge.

Note

in the end, this does not change the performance too much. However, the size of the memory required to compute n blocks is dramatically smaller with this change, and allows the proving of bigger data, which might have been impossible before.