Introduces the Allocator API in `dart:ffi`.
This CL does not yet roll `package:ffi` to use `Allocator`, because that
breaks the checked in Dart in Fluter in g3. Instead, this coppies
`_MallocAllocator` from `package:ffi` into the ffi tests for testing.
This CL does not yet migrate off `allocate` and `free` in the SDK. That
is done in a dependent CL.
Issue: https://github.com/dart-lang/sdk/issues/44621
Issue: https://github.com/dart-lang/sdk/issues/38721
TEST=tests/ffi/allocator_test.dart
TEST=tests/ffi/calloc_test.dart
TEST=tests/ffi/vmspecific_static_checks_test.dart
Change-Id: I173e213a750b8b3f594bb8d4fc72575f2b6b91f7
Cq-Include-Trybots: luci.dart.try:vm-precomp-ffi-qemu-linux-release-arm-try,analyzer-analysis-server-linux-try,analyzer-linux-release-try,analyzer-nnbd-linux-release-try,front-end-linux-release-x64-try,front-end-nnbd-linux-release-x64-try,benchmark-linux-try,dart-sdk-linux-try,pkg-linux-release-try,vm-ffi-android-release-arm-try,vm-ffi-android-release-arm64-try,vm-kernel-nnbd-win-debug-x64-try,vm-kernel-win-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177705
Reviewed-by: Clement Skau <cskau@google.com>