Environ shim
Remake of https://github.com/rust-lang/miri/pull/1147. There are three main problems with this:
1. For some reason `update_environ` is not updating `environ` when `setenv` or `unsetenv` are called. Even then it works during initialization.
2. I am not deallocating the old array with the variables in `update_environ`.
3. I had to store the `environ` place into `MemoryExtra` as a field to update it. I was thinking about changing `extern_statics` to store places instead of `AllocID`s to avoid this.
@RalfJung
574d81ce79