[fbl] Make fbl::unique_ptr an alias of std::unique_ptr

fbl::unique_ptr differed from std::unique_ptr in a handful of ways.

- fbl::unique_ptr supported the fbl::Recycler technology. This had no
  clients (all clients use fbl::RefPtr instead).

- The deleter is differently named, and so some friend declarations
  needed to be tweaked.

- The constructors and conversions are slightly different. This
  manifested as a need to explicitly provide template arguments in a
  handful of places. In general, std:: allows more conversions than
  fbl:: did.

Bug: ZX-3357 #comment fbl::unique_ptr alias
Bug: ZX-3837 #done
Change-Id: I43b099e7e2905b87c3bc3ad493a30a9f05add4ba