Function encode_currency_code 

pub fn encode_currency_code(code: &str) -> Result<[u8; 20], &'static str>

Encode a 3-character currency code for XRPL.

XRPL currency codes are 20 bytes:

• Standard (3-char): 12 zero bytes + 3 ASCII bytes + 5 zero bytes
• Non-standard (40-hex): raw 20 bytes