Implement CHARVAL function (#1701)

test/asm/charval.asm

@@ -0,0 +1,23 @@
+charmap "a", 1
+charmap "b", 2, 3
+charmap "cdef", 4
+charmap "ghi", 5, 6, 7, 8, 9
+charmap "jkl", 123, 456, 789
+charmap "mno", 123456789
+
+assert charval("a", 0) == 1
+assert charval("a", -1) == 1
+assert charval("b", 0) == 2
+assert charval("b", 1) == 3
+assert charval("b", -1) == 3
+assert charval("b", -2) == 2
+assert charval("cdef", 0) == 4
+assert charval("ghi", 2) == charval("ghi", -3)
+assert charval("jkl", -1) == 789
+assert charval("mno", 0) == 123456789
+
+assert charval("abc", 0) == 0
+assert charval("cd", 1) == 0
+assert charval("xyz", 2) == 0
+assert charval("ghi", -10) == 5
+assert charval("ghi", 10) == 0

test/asm/charval.err

@@ -0,0 +1,11 @@
+error: charval.asm(19):
+    CHARVAL: No character mapping for "abc"
+error: charval.asm(20):
+    CHARVAL: No character mapping for "cd"
+error: charval.asm(21):
+    CHARVAL: No character mapping for "xyz"
+warning: charval.asm(22): [-Wbuiltin-args]
+    CHARVAL: Index starts at 0
+warning: charval.asm(23): [-Wbuiltin-args]
+    CHARVAL: Index 10 is past the end of the character mapping
+error: Assembly aborted (3 errors)!