process: deprecate Child stdio accessors in favor of pub fields

[ipetkov]

Motivation

The current strategy of accessing Child stdio handles via accessors does not allow for mutably borrowing over multiple stdio handles.

Solution

Deprecate stdio accessors in favor of public fields. The Child struct still has a private member, so we can add additional fields without breaking changes

Fixes #2009

[carllerche]

Looks great to me 👍 thanks for the fast turn around. I requested a review from @sfackler (just to make sure he doesn't see any backwards compat issues).

[carllerche]

Thanks 👍