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, 10 insertions, 9 deletions

diff --git a/libbuild2/lexer.hxx b/libbuild2/lexer.hxx
index 4371206..e913829 100644
--- a/libbuild2/lexer.hxx
+++ b/libbuild2/lexer.hxx
@@ -26,14 +26,15 @@ namespace build2
   // mode we don't treat certain characters (e.g., `+`, `=`) as special so
   // that we can use them in the variable values, e.g., `foo = g++`. In
   // contrast, in the variable mode, we restrict certain character (e.g., `/`)
-  // from appearing in the name. The values mode is like value but recogizes
-  // `,` as special (used in contexts where we need to list multiple
-  // values). The attributes/attribute_value modes are like values where each
-  // value is potentially a variable assignment; they don't treat `{` and `}`
-  // as special (so we cannot have name groups in attributes) as well as
-  // recognizes `=` and `]`. The subscript mode is like value but doesn't
-  // treat `{` and `}` as special and recognizes `]`. The eval mode is used in
-  // the evaluation context.
+  // from appearing in the name. Additionally, in the variable mode we
+  // recognize leading `\` as the beginning of the escape sequent ($\n). The
+  // values mode is like value but recogizes `,` as special (used in contexts
+  // where we need to list multiple values). The attributes/attribute_value
+  // modes are like values where each value is potentially a variable
+  // assignment; they don't treat `{` and `}` as special (so we cannot have
+  // name groups in attributes) as well as recognizes `=` and `]`. The
+  // subscript mode is like value but doesn't treat `{` and `}` as special and
+  // recognizes `]`. The eval mode is used in the evaluation context.
   //
   // A number of modes are "derived" from the value/values mode by recognizing
   // a few extra characters:
@@ -262,7 +263,7 @@ namespace build2
     // been "expired" from the top).
     //
     virtual token
-    word (state current, bool separated);
+    word (const state& current, bool separated);
 
     // Return true in first if we have seen any spaces. Skipped empty lines
     // don't count. In other words, we are only interested in spaces that are