Function derive_mapping_slot 

pub fn derive_mapping_slot(
    base_slot: &[u8],
    fragments: &[KeyFragment],
) -> [u8; 32]

Derive the storage slot for a mapping entry given the base slot and key fragments.

Each key fragment is serialised with a 4-byte little-endian length prefix to avoid ambiguity between {a, bc} and {ab, c} style encodings.