* Make `alloc` AIR instructions call `resolveTypeLayout`.
 * `Sema.zirResolveInferredAlloc` now calls `requireRuntimeBlock` in the
   case that it operates on a non-comptime instruction.
 * `Type.abiSize` and `Type.abiAlignment` now return 0 for `void`
 * Sema: implement `resolveTypeFields` for unions.
 * LLVM Backend: support `ptr_elem_ptr` when the element type is 0-bit.
 * Type: improve `abiAlignment` implementation for structs to properly
   handle fields with non-default alignment.
 * Value: implement hashing array, vector, and structs.