support -x 0 to print encoding name.

[katrinafyi]

this is very useful when looking for the name of an encoding, without cluttering the output with the disassembly trace.

the default debug_level has been lowered to -1 to support -x 0 as a non-default level. we cannot print by default since that would clutter stdout when used as a library.

example:

$ echo ':sem A64 0xd4207d00' | dune exec asli -- -x 0
Decoding instruction A64 d4207d00     
Disasm: aarch64_system_exceptions_debug_breakpoint
BTypeCompatible = TRUE ;
assert FALSE ;