Implement value typing, null support via value attributes

For example:

v = [null]
v = [string] abc
v += ABC # abcABC

1 files changed, 1 insertions, 1 deletions

diff --git a/build2/b.cxx b/build2/b.cxx
index 44fa032..f9c3683 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -819,7 +819,7 @@ main (int argc, char* argv[])
             }
 
             value& v (p.first);
-            v.assign (names (o.val), o.var); // Original var for diagnostics.
+            v = o.val;
             first = false;
           }