Provide an API to extract fields from Command builder

[euclio]

This is now implemented on nightly via #77029.

Summary
The following accessors are available on Command behind the #![feature(command_access)] gate:

• Command::get_program
• Command::get_args
• Command::get_envs
• Command::get_current_dir

Unresolved issues
Some concerns I had with the implementation, but probably not important:

• Values with NULs on Unix will be returned as "<string-with-nul>". I don't think it is practical to avoid this, since otherwise a whole separate copy of all the values would need to be kept in Command.
• Does not handle arg0 on Unix. This can be awkward to support in get_args and is rarely used. I figure if someone really wants it, it can be added to CommandExt as a separate method.
• Does not offer a way to detect env_clear. I'm uncertain if it would be useful for anyone.
• Does not offer a way to get an environment variable by name (get_env). I figure this can be added later if anyone really wants it. I think the motivation for this is weak, though. Also, the API could be a little awkward (return a Option<Option<&OsStr>>?).
• get_envs could skip "cleared" entries and just return &OsStr values instead of Option<&OsStr>. I'm on the fence here. My use case is to display a shell command, and I only intend it to be roughly equivalent to the actual execution, and I probably won't display None entries. I erred on the side of providing extra information, but I suspect many situations will just filter out the Nones.
• Could implement more iterator stuff (like DoubleEndedIterator).

Original issue below

The the std::process::Command builder is useful for building up a Command in a cross-platform way. I've found that it would be useful to extract the name, args, environment, etc. of a Command they have been set.

There are at least two places in the Rust compiler that would benefit from such an API. Instead, the authors have had to resort to wrappers instead of using Command directly.

https://github.com/rust-lang/rust/blob/master/src/tools/compiletest/src/runtest.rs#L1527

https://github.com/rust-lang/rust/blob/master/src/librustc_trans/back/command.rs

[lukaslueg]

Related to #42200

[mackwic]

Let's say I am ok to implement this, how should I proceed ? Directly with a PR or with an RFC first ?

[dtolnay]

Seems reasonable. I would be interested in seeing this explored in a PR.

[ebkalderon]

I am working on a crate that would directly benefit from this. Is anyone still working on this feature? If not, would someone mind mentoring me to open a PR?

[ebkalderon]

Also, I have a tangential question not related to this issue, but might be raised as a result of introducing this feature: is there any way in the standard library to check whether a given std::process::Stdio is set to inherit, piped, or null? If we add the ability to extract these fields from the Command builder, I think it makes sense to add the ability to inspect the values at run-time as well.