Summary:
shergin mentioned that he'd like to move away from RTTI a bit and use explicit key strings for context container instances rather than relying on the `typeid`, so this does this.
We also fatal with a useful error message if we get a collision, rather than failing silently.
Reviewed By: shergin
Differential Revision: D13384308
fbshipit-source-id: 0b06d7555b082be89e8f130c23e94be99749a7a3