Implement BYTELEN and STRBYTE (#1744)

test/asm/bytelen-strbyte.asm

@@ -0,0 +1,36 @@
+assert bytelen("") == 0
+assert bytelen("ABC") == 3
+assert strbyte("ABC", 0) == $41
+assert strbyte("ABC", -1) == $43
+
+charmap "ABC", 42
+assert bytelen("ABC") == 3
+
+; characters:
+; 1: U+72AC kanji (0xE7 0x8A 0xAC)
+; 2: U+1F499 emoji (0xF0 0x9F 0x92 0x99)
+; 3: U+0021
+def utf8 equs "犬💙!"
+assert bytelen(#utf8) == 8
+assert strbyte(#utf8, 0) == $e7
+assert strbyte(#utf8, 4) == $9f
+assert strbyte(#utf8, -2) == $99
+assert strbyte(#utf8, -1) == $21
+
+; characters:
+; 1: U+0041 A
+; 2: U+0020 space
+; 3: invalid byte 0xFE
+; 4: invalid byte 0x81
+; 5: invalid byte 0xFF
+; 6: U+0020 space
+; 7: U+6F22 kanji (0xE6 0xBC 0xA2)
+def invalid EQUS "A <20><><EFBFBD> 漢"
+assert bytelen(#invalid) == 9
+assert strbyte(#invalid, 0) == $41
+assert strbyte(#invalid, 4) == $ff
+assert strbyte(#invalid, 8) == $a2
+
+; out of bounds
+assert strbyte("abc", -10) == $61 ; -10 clamped to 0
+assert strbyte("abc", 10) == 0