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

This commit adds `env_insert` and `env_remove` methods to the `Command`
builder, easing updates to the environment variables for the child
process. The existing method, `env`, is still available for overriding
the entire environment in one shot (after which the `env_insert` and
`env_remove` methods can be used to make further adjustments).

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`.
While I doubt that it was intended behavior, let's see what happens!

In general, it should probably continue to work, but it's also probably best to not nuke things like HOME and USER.