Fix various decoding issues.

[partim]

This PR fixes a number of decoding issues. Specifically:

• error out rather than panic when a nested value has a greater length than allowed by the outer value,
• check that there is enough data available before skipping over a primitive value’s content,
• checks that enough data is available before trying to parse a tag value,
• checks for correct encoding of bit strings: don’t allow the number of unused bits to be greater than 7 and that they are zero for an empty bit string,
• checks for correct encoding of object identifiers: they cannot be empty and the last byte must have bit 7 cleared.

The PR adds tests for these cases. It also adds a number of fuzzing targets to be used with cargo fuzz.

This PR provides the fixes for CVE-2023-39914.