minimize heap allocations in public API

[sdemos]

The API exposed by the library uses a lot of Vec<u8> and String instead of arrays and str. We should investigate if it makes sense to convert them to minimize heap allocations, at least for functions that provide the core functionality of the library.

Functions that allocate on the heap which should be investigated -

• read_keys
• from_rsa
• from_dsa
• data
• to_key_format
• fingerprint
• to_fingerprint_string
• fingerprint_md5
• to_fingerprint_md5_string