Add support for evaluation context

For now it acts as just the value mode that can be enabled anywhere
variable expansion is supported, for example:

(foo=bar):

And the primary use currently is to enable/test quoted and indirect
variable expansion:

"foo bar" = FOO BAR
print $"foo bar"    # Invalid.
print $("foo bar")  # Yeah, baby.

foo = FOO
FOO = foo
print $($foo)

Not that you should do something like this...

1 files changed, 3 insertions, 0 deletions

diff --git a/build/parser b/build/parser
index b5155be..0678a62 100644
--- a/build/parser
+++ b/build/parser
@@ -77,6 +77,9 @@ namespace build
     variable_name (names_type&&, const location&);
 
     names_type
+    eval (token&, token_type&);
+
+    names_type
     names (token& t, token_type& tt)
     {
       names_type ns;