Fix string concatenation on local constants

Fixes #301. Includes a test.

Note: the following still does not work inside of a function, although I'm not sure what to do about it (other than reworking local constants to use #define + #undef like non-local constants do):

	const string str1 = "Hello";
	const string str2 = ", ";
	const string str3 = "world!";
	const string merged = str1 + str2 + str3;