Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stubs/pyre-extensions/METADATA.toml

@@ -0,0 +1,2 @@
+version = "0.1"
+python2 = true

stubs/pyre-extensions/pyre_extensions.pyi

@@ -0,0 +1,7 @@
+from typing import Any, List, Optional, Type, TypeVar
+
+_T = TypeVar("_T")
+
+def none_throws(optional: Optional[_T], message: str = ...) -> _T: ...
+def safe_cast(new_type: Type[_T], value: Any) -> _T: ...
+def ParameterSpecification(__name: str) -> List[Type[Any]]: ...