Avoid single character String allocations when appending characters

Removed calls to String::chr() when appending characters to Strings in Expression, Resource, and VariantParser, to avoid creating temporary Strings for each character.  Also updated the Resource case to resize String up front, since size is known.

core/math/expression.cpp

@@ -408,7 +408,7 @@ Error Expression::_get_token(Token &r_token) {
 						if (reading == READING_DONE) {
 							break;
 						}
-						num += String::chr(c);
+						num += c;
 						c = GET_CHAR();
 						is_first_char = false;
 					}
@@ -435,7 +435,7 @@ Error Expression::_get_token(Token &r_token) {
 					cchar = GET_CHAR();
 
 					while (is_unicode_identifier_continue(cchar)) {
-						id += String::chr(cchar);
+						id += cchar;
 						cchar = GET_CHAR();
 					}