Switch to an Iterator interface.

This is to solve an issue where writers returned by the Formatter
were often stateful, but this fact was not obvious to the API consumer,
and failed in interesting ways.

lexer_test.go

@@ -47,7 +47,6 @@ func TestSimpleLexer(t *testing.T) {
 		{Whitespace, " "},
 		{LiteralString, "10"},
 		{Whitespace, "\n"},
-		{EOF, ""},
 	}
 	require.Equal(t, expected, actual)
 }