The methods above_of(), left_of() and above_left_of() returned the
matching block by value, or a default block if the offset resulted in a
block outside boundaries.

The Block structure is quite big (std::mem::size_of::<Block>() == 120).
For reading a field, it is probably not optimal to return a whole Block
copy or a new default block (although the compiler might optimize such
accesses).

Moreover, the boundaries checks were often redundant, because already
done by the callers.

Instead, let the callers check boundaries and return a reference to the
matching block.