README: Add instructions for building on windows.
Implementation of common traits for MaybeOwnedVector
auto merge of #15210 : luqmana/rust/windoc, r=alexcrichton

Getting rust to build on windows can be a bit annoying in setting up all the toolchains and whatnot. The whole process is made much easier by using msys2 so let's document that prominently right on the README.
auto merge of #15241 : arjantop/rust/maybeownedvector-improvements, r=alexcrichton
librustc: Remove the fallback to `int` for integers and `f64` for
floating point numbers for real.

This will break code that looks like:

    let mut x = 0;
    while ... {
        x += 1;
    }
    println!("{}", x);

Change that code to:

    let mut x = 0i;
    while ... {
        x += 1;
    }
    println!("{}", x);

Closes #15201.

[breaking-change]