We were only keeping track of `RawSyntax` node IDs to incrementally transfer a syntax tree via JSON. However, AFAICT the incremental JSON transfer option has been superceeded by `SyntaxParseActions`, which are more efficient.
So, let’s clean up and remove the `RawSyntax` node ID and JSON incremental transfer option.
In places that still need a notion of `RawSyntax` identity (like determining the reused syntax regions), use the `RawSyntax`’s pointer instead of the manually created ID.
In `incr_transfer_round_trip.py` always use the code path that uses the `SyntaxParseActions` and remove the transitional code that was still using the incremental JSON transfer but was never called.
It was originally designed for faster trasmission of syntax trees from
C++ to SwiftSyntax, but superceded by the CLibParseActions. There's no
deserializer for it anymore, so let's just remove it.
Use `io.open` to open the files with the correct encoding and to allow
proper newline specification. This allows the script to be python 2 and
python 3 compatible.
Enhances `swift-syntax-test` to output the parser diagnostics so we can verify that
if the incremental parse resulted in parser diagnostics, those diagnostics were also emitted during the full parse.
Use the python library `difflib` which is part of a standard python
installation to generate the diff. This allows us to run the tool on
Windows wihtout needing external tools installed.
