Expand environment-handling in folly

Expand environment-handling in folly

Summary:
Extract the environment-as-STL-map logic out of `test::EnvVarSaver` and into a
separate class `experimental::EnvironmentState` so that other code which needs
to manipulate the environment can do so more easily.

Add routines to set the process environment from the state of an
`EnvironmentState`'s map, and to extract the environment in the forms required
by `Subprocess` and the UNIX routines `execve` etc.

Reviewed By: yfeldblum

Differential Revision: D4713307

fbshipit-source-id: 6b1380dd29b9ba41c97b886814dd3eee91fc1c0f