Fixed option_env! type

The type of the result of option_env! was not fully specified in the
None case, leading to type check failures in the case where the variable
was not defined (e.g. option_env!("FOO").is_none()).
auto merge of #8440 : sfackler/rust/env-fix, r=pcwalton

The type of the result of option_env! was not fully specified in the
None case, leading to type check failures in the case where the variable
was not defined (e.g. option_env!("FOO").is_none()).

Also cleaned up some compilation warnings.