Fix tense inconsistencies in doc comments

I see some inconsistencies in tense (present and past) in some doc comments (currently detected in generic_cube.rs).
E.g. 
```rust
/// Maps over the pieces of the cube, replacing each piece
/// according to the given mask function. 
fn mask(&self, mask: &dyn Fn(i32, Face) -> Face) -> Self;

/// Apply a move to a cube.
/// ...
fn apply_move(&self, mv: Move) -> Self;
```
