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, 15 insertions, 6 deletions

diff --git a/libbuild2/token.cxx b/libbuild2/token.cxx
index ab14388..cc102cc 100644
--- a/libbuild2/token.cxx
+++ b/libbuild2/token.cxx
@@ -29,21 +29,30 @@ namespace build2
         os << (r ? "\n" : "<newline>");
         break;
       }
-    case token_type::pair_separator:
+    case token_type::word:
       {
         if (r)
-          os << t.value[0];
+          os << t.value;
         else
-          os << "<pair separator " << t.value[0] << ">";
+          os << '\'' << t.value << '\'';
 
         break;
       }
-    case token_type::word:
+    case token_type::escape:
       {
         if (r)
-          os << t.value;
+          os << '\\' << t.value;
         else
-          os << '\'' << t.value << '\'';
+          os << "<escape sequence \\" << t.value << ">";
+
+        break;
+      }
+    case token_type::pair_separator:
+      {
+        if (r)
+          os << t.value[0];
+        else
+          os << "<pair separator " << t.value[0] << ">";
 
         break;
       }