Removed prelude::* from libstd files.

This replaces the imports from the prelude with the re-exported symbols.
Cleaned up imports per coding standards.

No functional changes; just style.
auto merge of #12075 : chromatic/rust/gh_11983_deglob_std, r=alexcrichton

This replaces the imports from the prelude with the re-exported symbols.
Added tests to make tidy
auto merge of #12055 : dguenther/rust/tidy_test, r=alexcrichton

This PR extends the tidy formatting check to rust files in the test folder. To facilitate this, a few flags were added to tidy:

* `xfail-tidy-cr` - Disables the check for CR characters for all following lines in the file
* `xfail-tidy-tab` - Disables the check for tab characters for all following lines in the file
* `xfail-tidy-linelength` - Disables the line length check for all following lines in the file

Checks should not have to be disabled often. I disabled line length checks in `debug-info` tests that use `debugger:` checks, but aside from that, there were relatively few exclusions. Running tidy on all the tests does slow down the formatting check, so it may be worth investigating further optimization.

cc #4534
Made `std::container::Set::is_superset` a default method.
Implemented container traits for Option

This allows treading a optional value as a one-element container.