utf8decoder: Use byte-sized byte argument

This prevents passing a negative value out of a signed char by accident.
Also renders some casts in the code superfluous.

test/asm/invalid-utf-8.asm

@@ -0,0 +1,5 @@
+; This test tries to pass invalid UTF-8 through a macro argument
+; to exercise the lexer's reportGarbageChar
+m:MACRO \1
+ENDM
+	m <EFBFBD><EFBFBD>

test/asm/invalid-utf-8.err

@@ -0,0 +1,5 @@
+ERROR: invalid-utf-8.asm(4) -> invalid-utf-8.asm::m(3):
+    Unknown character 0xCF
+ERROR: invalid-utf-8.asm(4) -> invalid-utf-8.asm::m(3):
+    Unknown character 0xD3
+error: Assembly aborted (2 errors)!