This makes it easy to only assign variables for the information you
need in the iterator. Before, we always forced you to take a match and
a range as the first two arguments even if you weren't using them.
We might have two edit sessions pointing to the same buffer, for
example if we have a split pane… So when we deserialize a buffer, we
always need to check that we don't already have an instance of that
buffer on the project. If we do, then we've already deserialized it
once so we don't need to worry about the saved text.
We still have a problem when deserializing previously unsaved buffers,
because we can't use the path to identify them.
Previously the line ending length was hard-coded to one which
would cause TextBuffer.scanInRange() to return incorrect results
since one character per line wasn't being accounted for.
Closes#428
