Improve escape sequence support

Specifically:

1. In the double-quoted strings we now only do effective escaping of the
   special `$("\` characters plus `)` for symmetry.

2. There is now support for "escape sequence expansion" in the form $\X where
   \X can be any of the C/C++ simple escape sequences (\n, \t, etc) plus \0
   (which in C/C++ is an octal escape sequence). For example:

   info "foo$\n$\tbar$\n$\tbaz"

   Will print:

   buildfile:1:1: info: foo
           bar
           baz

1 files changed, 1 insertions, 1 deletions

diff --git a/libbuild2/test/script/lexer.hxx b/libbuild2/test/script/lexer.hxx
index 993a9db..39b950a 100644
--- a/libbuild2/test/script/lexer.hxx
+++ b/libbuild2/test/script/lexer.hxx
@@ -77,7 +77,7 @@ namespace build2
         next_description ();
 
         virtual token
-        word (state, bool) override;
+        word (const state&, bool) override;
       };
     }
   }