Report garbage chars as their bytes; don't try decoding them as UTF-8

This decoding required high lookahead, and was not even
consistently useful (the `garbage_char` test case was not
valid UTF-8 and so did not benefit from `reportGarbageChar`).

This limits UTF-8 handling to the `STRLEN` and `STRSUB`
built-in functions, and to charmap conversion.

test/asm/null-in-macro.err

@@ -1,3 +1,3 @@
 ERROR: null-in-macro.asm(4) -> null-in-macro.asm::foo(2):
-    Unknown character 0x00
+    Unknown character '\x00'
 error: Assembly aborted (1 error)!