Function decode 

pub fn decode(s: &str) -> Result<Vec<u8>, SExprError>

Decode a lowercase (or mixed-case) hex string into bytes.

Returns SExprError::Parse on:

• odd-length input (can’t chunk into byte pairs),
• non-ASCII bytes,
• non-hex digits.

The empty string decodes to an empty Vec<u8> — valid degenerate.