Support crate_features and proc macros in doctests

[bsilver8192]

Previously setting features for the doctests themselves wasn't supported at all, and attempting to use a proc macro would fail. I needed both of these together to write a test.

This is the last of my changes to get rust_doc_test working for all of my crates at work.

[bsilver8192]

The tests fail on Windows, I think it's because rustdoc_test_writer's escaping doesn't work. Unfortunately I don't know enough Windows shell scripting to fix it. What should I do? I see these options:

• Somebody else fixes the Windows code before merging, either in this PR or a separate one
• File a bug and disable the problematic test rules on Windows
• File a bug, use select to avoid setting the cfg for Windows, and comment out the problematic tests
• File a bug, use select to avoid setting the cfg for Windows, and cfg(not(target_os = "windows")) t
• Split out the crate_features support for a separate PR, and fork rustdoc_lib.rs into proc_macro and non-proc_macro variants without any cfg

[bsilver8192]

@UebelAndre I need help on this. I added a test for the escaping (and found+fixed a bug on Linux), but I have no idea how to do this on Windows. Nothing I read online seems to work the way it's claimed, but it's also hard to confirm what's happening without a local machine to experiment interactively. Seems like there's 2 or 3 nested layers of parsing that all have to be satisfied without escape characters that they strip off, and it may be impossible to do robustly.