This fixes a logic error on master that uses `Anchor::offset` as an
offset into the fragment. This is wrong because fragments could be
subject to further splits, thus making the relative offset potentially
invalid.

This avoids unnecessary lookups in the buffer tree when the anchor's
position has already been retrieved since the last edit.

We are representing the cache as a RefCell to avoid the need to acquire
a mutable reference to the buffer even if we just need to query it. If
the cache is already borrowed for some reason, we will simply skip
writing to it.

This will allow to efficiently clone the FragmentId (i.e., without heap
allocations) for comparison purposes when it's used during a tree
traversal.

`lazy_static!` requires variables to be thread-safe (for good reasons),
so we have also replaced `Rc` with an `Arc`. This doesn't seem to affect
performance in a significant way.

When moving cursors, we heavily rely on `Buffer::len_for_row` to
understand when such cursors should wrap. This method is implemented by
traversing the tree twice: the first traversal gets the offset of the
beginning of the requested line, whereas the second traversal gets the
offset of the line next to the requested one. Then, the two offsets are
subtracted to obtain the length of the line.

With this commit we start caching the mapping between offsets and
points. This allows to skip the two traversals and retrieve the length
of a line in constant time if the line length had been computed before
and no edit has occurred since then.