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.

1 files changed, 0 insertions, 1 deletions

diff --git a/src/test/ui/iterators/iter-count-overflow-ndebug.rs b/src/test/ui/iterators/iter-count-overflow-ndebug.rs
index b755bb554f4..c38755db87e 100644
--- a/src/test/ui/iterators/iter-count-overflow-ndebug.rs
+++ b/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() {