Add support for new Unicode string escape \u

[benhoyt]

See similar onetrueawk and Gawk changes:

• https://github.com/onetrueawk/awk/commit/ac3084de9b64d0c838e520ea1ebdb2e1bde87b3f
• https://git.savannah.gnu.org/cgit/gawk.git/commit/?id=1bc73bfb10f0600aaccca7220b5200e220f9a71a

Also, in lexer tests, stop after first error.

Fixes #210