Makes <<__Memoize>> work for functions with args of certain types

Summary: Right now, this only works with arguments whose types can safely be used as array keys without conversion. Arbitrary params will come in the next diff.

Reviewed By: @jano

Differential Revision: D1515148
Fix inst_meth example in hhi comment

Summary: `inst_meth` doesn't work on `private` anything due to runtime
restrictions.

Reviewed By: @pikatchu, @elgenie

Differential Revision: D1531767