`CBOR::StackError` is not rescued by `COSE::Key.cbor_decode`

[santiagorodriguez96]

The cbor gem caps nesting depth at 128 (MSGPACK_UNPACKER_STACK_CAPACITY) and raises CBOR::StackError. That class isn't in cbor_decode's rescue list, so it escapes as a cbor gem error instead of COSE::MalformedKeyError:

require "cose/key"
require "uri"   # so the NameError in #82 doesn't mask this

deep = 0
129.times { deep = [deep] }

COSE::Key.deserialize(deep.to_cbor)
# CBOR::StackError: stack level too deep

Callers shouldn't have to rescue cbor gem classes. We should add CBOR::StackError to the list of rescued exceptions so that COSE::MalformedKeyError is raised instead.