generics: fix errors & simplify

vlib/compiler/cgen.v

@@ -16,6 +16,7 @@ struct CGen {
 	is_user      bool
 mut:
 	lines        []string
+	lines_extra  []string
 	typedefs     []string
 	type_aliases []string
 	includes     []string
@@ -102,6 +103,7 @@ fn (g mut CGen) resetln(s string) {
 fn (g mut CGen) save() {
 	s := g.lines.join('\n')
 	g.out.writeln(s)
+	g.out.writeln(g.lines_extra.join('\n'))
 	g.out.close()
 }