std::io::pipe needs to be useful

[alexcrichton]

Currently this module supports consuming a file descriptor to make an I/O object, but it provides no method of creating the file descriptor itself. The current usage of the module is essentially tailored for whatever std::io::process needed to get implemented. Some things I believe need to improve:

• Is PipeStream the right name?
• PipeStream should be constructable with a function that takes no arguments
• Should the underlying file descriptor be exposed through a method?

I'm nominating this, but I don't view this as a super-critical piece of infrastructure. My current feeling is that it's "good as is" except for the name PipeStream which I can learn to live with. The other extra methods can be added later. That being said, this module needs a close look to ensure there's nothing backwards incompatible we'll want to live with.

[eddyb]

I think MemoryMap has the same file descriptor issue.

[pnkfelix]

Marking P-low, not 1.0 issue.