`compile_fail` and `should_panic` examples are tested wherever possible

[Cerber-Ursi]

A couple of days ago there was a question on URLO, which uncovered the fact that some compile-fail cases in the Book are not marked as such. After some research, I've decided to propose the following:

• Use rustdoc annotations (compile_fail and should_panic) instead of custom does_not_compile and panics.
• Drop the ignore annotation wherever possible, so that the examples, including non-compiling or panicking ones, are checked to behave as expected.
• Add should_panic or compile_fail annotations wherever they seem to be expected.

Note that some failing examples can't be tested, because the failing code is #[cfg(test)], and this property is not set for doctests. These examples are, however, explicitly marked as both compile_fail or should_panic and ignore, so that it would be now easy to grep for them during future changes.