RFC 2070 part 1: PanicInfo and Location API changes
This implements part of https://rust-lang.github.io/rfcs/2070-panic-implementation.html
Tracking issue: https://github.com/rust-lang/rust/issues/44489
* Move `std::panic::PanicInfo` and `std::panic::Location` to a new `core::panic` module. The two types and the `std` module were already `#[stable]` and stay that way, the new `core` module is `#[unstable]`.
* Add a new `PanicInfo::message(&self) -> Option<&fmt::Arguments>` method, which is `#[unstable]`.
* Implement `Display` for `PanicInfo` and `Location`