Made types used by Symfony compatible with the ones of Hack

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

PHP supports several ways to express types: like Boolean/bool or integer/int. Hack only supports one of them, so this PR proposes to use the Hack type to make Symfony a bit more "compatible" with Hack (gradual upgrade ;)).