Fix lexer to not produce bad unicode escape values

There were a couple of issues in the lexer unicode escape code.
Unicode escape sequences must always start with an opening curly
bracket (and end with a closing one). Underscores are not allowed as
starting character. And the produced values must be unicode scalar
values, which excludes surrogate values (D800 to DFFF) or values
larger than 10FFFF.

Also try to recover more gracefully from errors by trying to skip past
any bad characters to the end of the escape sequence.

Test all of the above in a new testcase unicode_escape.rs.

0 files changed, 0 insertions, 0 deletions