The existing round trip test was testing that we can pass through a set
of tokens through the AST entirely unmodified.
This new round-trip test passes the source through the Format function
and then checks that it has the same semantics, by evaluating it both
before and after and expecting an identical set of values.
Previously we tolerated EOF as an alias for newline, but a file without
a newline at the end is a edge case primarily limited to contrived
examples in unit tests, and requiring it simplifies tasks such as code
generation in zclwrite, since we can always assume that every block item
comes with a built-in line terminator.
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.
