This adds support to `quote_expr!` and friends for round-trip hygienic
preservation of Ident.
Here are the pieces of the puzzle:
* adding a method for encoding Ident for re-reading into token tree.
* Support for reading such encoded Idents in the lexer. Note that one
must peek ahead for MOD_SEP after scan_embedded_hygienic_ident.
* To ensure that encoded Idents are only read when we are in the midst
of expanding a `quote_expr` or similar, added a
`read_embedded_ident` flag on `StringReader`.
* pprust support for encoding Ident's as (uint,uint) pairs (for hygiene).
I chose to make two of them because I wanted something close to an
"end-to-end" test (*), but at the same time I wanted a test that
would run on Windows (**).
(*) The run-make test serves as the end-to-end: It constructs an input
that is trying to subvert the hack and we are going to check that it
fails in the attempt).
(**) The compile-fail-fulldeps test serves as a more narrow test that
will be tested on all platforms. It also attempts to subvert the
hack, testing that when you use `new_parser_from_tts`, the resulting
parser does not support reading embedded Idents.
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