Implement ++ operator for string concatenation (#1698)

test/asm/string-concat.asm

@@ -0,0 +1,26 @@
+SECTION "test", ROM0
+
+MACRO test
+	assert !strcmp(\1, \2)
+ENDM
+
+test "a"++"b", "ab"
+test "a"++""++"b", "ab"
+test "a"++"b", strcat("a", "b")
+test "a"++"b"++"c", strcat("a","b","c")
+test "" ++ "", ""
+test strupr("a") ++ strlwr("B"), "Ab"
+
+def str equs "hi"
+test #str ++ strupr(#str), "hiHI"
+test "a" ++ """b""" ++ strupr("c") ++ strslice(#str, 0, 0), "abC"
+
+charmap "a", 1
+charmap "b", 2
+charmap "ab", 12
+assert "a" + "b" == 3
+assert "a" ++ "b" == 12
+
+; errors
+assert 2 ++ 2 == 4
+ld a, [hl++]