The native parser's ranges don't include any surrounding comments, so we
need to do a little more work to pick them out of the surrounding token
sequences.
This just takes care of _lead_ comments, which are those that appear as
whole line comments above the item in question. Line comments, which
appear after the item on the same line, will follow in a later commit.
This now allows for round-tripping some input from bytes to tokens and
then back to bytes again. With no other changes, we expect this to produce
an identical result.