core: Improve ID hex formatting

[robin-nitrokey]

This patch tries to improve the confusing situation with ID hex formatting:

• The original Id::hex implementation (also used in Id::hex_path) skipped inner zero bytes so that different numbers are mapped to the same hex string. We need to keep this behavior for compatibility. But to indicate this flaw and avoid usage in new code, this PR removes Id::hex and renames Id::hex_path to Id::legacy_hex_path. Id::hex_path is kept as a deprecated alias for easier migration.
• There already is the Id::hex_clean function that correctly formats numbers with inner zero bytes. This PR also adds Id::clean_hex_path that can be used for paths in new code.
• The old Id::hex_clean function skipped all zero bytes, including the trailing zero. As it is currently only used in the Debug implementation, this PR changes it to format 0 as "00".
• This PR also adds unit tests for these three functions and removes unnecessary buffer allocations in the implementation.
• As a side effect, this allows us to drop the public heapless dependency in trussed-core. Not really important right now, but will be useful once we can upgrade heapless-bytes.