Escape sysroot and manifest paths to remove whitespaces

[AxelMontini]

Hopefully fixes #206

Fixes an error on windows where paths with whitespaces don't get escaped at all, causing the error error: multiple input filenames provided. In my case it happens because my Windows username is "Axel Montini" and xargo doesn't like the space in the middle.
This commit adds a function in utils to escape a string and replace spaces with %20 on windows or \ on other operative systems.
I haven't tested this on a Unix machine, but it seems to work on windows.

[phil-opp]

Hi @AxelMontini! Thank you for tackling #206!

It seems like xargo is no longer maintained, but we have an active fork at rust-osdev/cargo-xbuild. Your PR looks useful to us, so I opened rust-osdev#6 as a copy of this PR. Is this ok with you that we merge it?

[AxelMontini]

Hi @phil-opp! Yes, you can merge it if you want, so we can finally resolve this issue.

[RalfJung]

These paths are used in RUSTFLAGS, right? Escaping won't help, unfortunately. Did you actually test this? I don't think this will work.

See rust-lang/cargo#6139 -- with RUSTFLAGS there just is no way to pass spaces to rustc.

[RalfJung]

Oh I see, I think this works on Windows because it actually replaces the space by a string that does not have a space. The unix version will definitely not work, there is still a space in the string and backslash does not help.

Can you update this PR to instead error on unix, pointing to rust-lang/cargo#6139 ? We just cannot support paths with spaces on that platform currently. We can do that on Windows though, which seems nice!

[phil-opp]

If I remember correctly, I removed this patch from cargo-xbuild again after it turned out that it wasn't really working (on Windows).

[RalfJung]

Okay, closing this PR then.
In principle I am open to a Windows-only solution to this problem, but then we should really have Windows CI as well to make sure this works.