Function map_entry_layout 

pub fn map_entry_layout(key: &Type, value: &Type) -> FixedLayout

Layout of one map<K, V> entry block. An entry is exactly a dense 2-field struct — key at implicit field id 0, value at id 1 — so every implementation derives the same key/value slot offsets, stride, and alignment from K and V alone, no schema type needed. slots[0] is the key offset, slots[1] the value; size is the entry stride.