Add sizeof, decltype, and declval snippets

UltiSnips/c.snippets

@@ -11,6 +11,10 @@ def complete(t, opts):
 	return '(' + '|'.join(opts) + ')'
 endglobal
 
+snippet sizeof "sizeof" i
+sizeof($1)$0
+endsnippet
+
 snippet i8 "int8_t" i
 int8_t
 endsnippet