Allow "bare" dynamic attrs

Now, in addition to a."${b}".c, you can write a.${b}.c (applicable
wherever dynamic attributes are valid).

Signed-off-by: Shea Levy <shea@shealevy.com>

src/libexpr/lexer.l

@@ -117,6 +117,8 @@ or          { return OR_KW; }
               return INT;
             }
 
+\$\{        { return DOLLAR_CURLY; }
+
 \"          { BEGIN(STRING); return '"'; }
 <STRING>([^\$\"\\]|\$[^\{\"]|\\.)+ {
               /* !!! Not quite right: we want a follow restriction on

src/libexpr/parser.y

@@ -509,6 +509,7 @@ attr
 
 string_attr
   : '"' string_parts '"' { $$ = $2; }
+  | DOLLAR_CURLY expr '}' { $$ = $2; }
   ;
 
 expr_list