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`.
This seems like another candidate for `env` over `env_set_all`