io::process::Command: add fine-grained env builder

This commit changes the `io::process::Command` API to provide
fine-grained control over the environment:

* The `env` method now inserts/updates a key/value pair.
* The `env_remove` method removes a key from the environment.
* The old `env` method, which sets the entire environment in one shot,
  is renamed to `env_set_all`. It can be used in conjunction with the
  finer-grained methods. This renaming is a breaking change.

To support these new methods, the internal `env` representation for
`Command` has been changed to an optional `HashMap` holding owned
`CString`s (to support non-utf8 data). The `HashMap` is only
materialized if the environment is updated. The implementation does not
try hard to avoid allocation, since the cost of launching a process will
dwarf any allocation cost.

This patch also adds `PartialOrd`, `Eq`, and `Hash` implementations for
`CString`.

[breaking-change]