Update Cargo and its dependencies

This'll probably have a bunch of build errors, so let's try and head those off
and find them sooner rather than later!

src/bootstrap/bootstrap.py

@@ -351,11 +351,6 @@ class RustBuild(object):
             with open(self.rustc_stamp(), 'w') as rust_stamp:
                 rust_stamp.write(self.date)
 
-            if "pc-windows-gnu" in self.build:
-                filename = "rust-mingw-{}-{}.tar.gz".format(
-                    rustc_channel, self.build)
-                self._download_stage0_helper(filename, "rust-mingw")
-
         if self.cargo().startswith(self.bin_root()) and \
                 (not os.path.exists(self.cargo()) or
                  self.program_out_of_date(self.cargo_stamp())):