Migrate `b256_to_ascii_bytes` from solana connector to sway libs

Migrate `b256_to_ascii_bytes` to sway-libs
It's important to do it before we audit predicate of solana connectors

https://github.com/FuelLabs/fuel-connectors/blob/1d90228e5425ab4dc8018f72e09797ef385f2ab6/packages/solana-connector/predicate/src/main.sw#L38-L51

Start with the linked implementation in the Solana connector at predicate/src/main.sw lines 38-51, then inspect sway-libs for the appropriate library location and conventions. Done means b256_to_ascii_bytes is available from sway-libs and the connector can use the migrated implementation before its predicate audit.

Written by the indexing model from the issue text.