Update game Reblox: Change block generation, add capability to "hold"

Blocks now are randomly picked from a pool that is reset when pool is empty instead of randomly picking from all possible blocks each time. By default, the pool size (set as a const variable in grid.rs) is 3 per block.

Can now hold the currently falling block to use later. This is done with the "h" key, and can only be used once until a block has been dropped, after which it can be used once again.

Bug fix: Resetting a game with "lines cleared" > 10 would result in invalid score display.

Also some minor refactoring.