Allow for splitting a `Pointer` by `Token` position (index)

[chanced]

Per our discussion, we need a way to index a token. With the index, at minimum, we need to be able to split a pointer at it, retrieve the token, and expose it via an iterator.

Once created, AssignError, ResolveError, and DeleteError need to be updated to include it, replacing offset.

/foo/bar/baz
↑   ↑   ↑
0   1   2