Introduce dedicated AST node for union

Although structs and unions have the same syntax and differ only in
the keyword, re-using the single syntax node for both of them leads to
confusion in practice, and propagates further down the hir in an
upleasent way.

Moreover, static and consts also share syntax, but we use different
nodes for them.

crates/ra_syntax/test_data/parser/inline/ok/0068_union_items.txt

@@ -1,5 +1,5 @@
 SOURCE_FILE@[0; 51)
-  STRUCT_DEF@[0; 12)
+  UNION_DEF@[0; 12)
     UNION_KW@[0; 5) "union"
     WHITESPACE@[5; 6) " "
     NAME@[6; 9)
@@ -9,7 +9,7 @@ SOURCE_FILE@[0; 51)
       L_CURLY@[10; 11) "{"
       R_CURLY@[11; 12) "}"
   WHITESPACE@[12; 13) "\n"
-  STRUCT_DEF@[13; 50)
+  UNION_DEF@[13; 50)
     UNION_KW@[13; 18) "union"
     WHITESPACE@[18; 19) " "
     NAME@[19; 22)

crates/ra_syntax/test_data/parser/ok/0035_weird_exprs.txt

@@ -1592,7 +1592,7 @@ SOURCE_FILE@[0; 3813)
       BLOCK@[2845; 2906)
         L_CURLY@[2845; 2846) "{"
         WHITESPACE@[2846; 2851) "\n    "
-        STRUCT_DEF@[2851; 2904)
+        UNION_DEF@[2851; 2904)
           UNION_KW@[2851; 2856) "union"
           WHITESPACE@[2856; 2857) " "
           NAME@[2857; 2862)