Qualify panic! as core::panic! in non-built-in core macros

Otherwise code like this

    #![no_implicit_prelude]

    fn main() {
        ::std::todo!();
        ::std::unimplemented!();
    }

will fail to compile, which is unfortunate and presumably unintended.

This changes many invocations of `panic!` in a `macro_rules!` definition
to invocations of `$crate::panic!`, which makes the invocations hygienic.

Note that this does not make the built-in macro `assert!` hygienic.

src/test/ui/iterators/iter-count-overflow-ndebug.rs

@@ -2,7 +2,6 @@
 // only-32bit too impatient for 2⁶⁴ items
 // compile-flags: -C debug_assertions=no -C opt-level=3
 
-use std::panic;
 use std::usize::MAX;
 
 fn main() {