feat: replace Schema with FieldLayout for contract internals

.changeset/grumpy-files-heal.md

@@ -0,0 +1,30 @@
+---
+"@latticexyz/cli": major
+"@latticexyz/protocol-parser": minor
+"@latticexyz/store": major
+"@latticexyz/world": major
+---
+
+1. Add `FieldLayout`, which is a `bytes32` user-type similar to `Schema`.
+
+Both `FieldLayout` and `Schema` have the same kind of data in the first 4 bytes.
+
+- 2 bytes for total length of all static fields
+- 1 byte for number of static size fields
+- 1 byte for number of dynamic size fields
+
+But whereas `Schema` has `SchemaType` enum in each of the other 28 bytes,
+`FieldLayout` has static byte lengths in each of the other 28 bytes.
+
+2. Replace `Schema valueSchema` with `FieldLayout fieldLayout` in Store and World contracts.
+
+`FieldLayout` is more gas-efficient because it already has lengths, and `Schema` has types which need to be converted to lengths.
+
+3. Add `getFieldLayout` to `IStore` interface.
+
+There is no `FieldLayout` for keys, only for values, because key byte lengths aren't usually relevant on-chain.
+You can still use `getKeySchema` if you need key types.
+
+4. Add `fieldLayoutToHex` utility to `protocol-parser` package.
+
+5. Add `constants.sol` for constants shared between `FieldLayout`, `Schema` and `PackedCounter`.