cbor: allow user to control nesting (#329)

* cbor: allow user to control nesting

 - Make the default read/write entrypoints allow infinite nesting.
 - Add {read,write}_nested() entrypoints that allow the crate user to
   control the depth of nesting that's allowed.
 - Along the way, convert the write[_nested] variants to return a
   `Result<(), EncoderError>` rather than a bool.  This exposes
   more failure information (and forces the caller to take notice
   of those tailures), and allows use of the ? operator.

* fixup: transmute error

Co-authored-by: kaczmarczyck <43844792+kaczmarczyck@users.noreply.github.com>

libraries/cbor/fuzz/fuzz_targets/fuzz_target_cbor.rs

@@ -8,7 +8,7 @@ use sk_cbor as cbor;
 fuzz_target!(|data: &[u8]| {
     if let Ok(value) = cbor::read(data) {
         let mut result = Vec::new();
-        assert!(cbor::write(value, &mut result));
+        assert!(cbor::write(value, &mut result).is_ok());
         assert_eq!(result, data);
     };
 });