Add #[must_use] to remaining core functions

[jkugelman]

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is everything remaining from the core crate.

Ignored by clippy for reasons unknown:

core::alloc::Layout   unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self;
core::any             const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str;

Ignored by clippy because of mut:

str   fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);

Ignored by clippy presumably because a caller might want `f` called for side effects. That seems like a bad usage of `map` to me.

core::cell::Ref<'b, T>   fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, T>;
core::cell::Ref<'b, T>   fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>);

Parent issue: #89692

r? @joshtriplett

[joshtriplett]

I left a comment on two functions that I think should not have must_use. r=me with those two dropped.

[jkugelman]

@joshtriplett What does r=me mean, by the way?

[joshtriplett]

@jkugelman If someone says "r=me with xyz fixed", that means "anyone with permission who comes along later is welcome to bors r=myusername if they see that xyz has been fixed; you don't need to wait for me to come back and re-review". So, for instance, if someone had come along after your change, before I did, they could have written (with an at-sign before bors) bors r=@joshtriplett. Given appropriate permissions, you could also do that yourself, on your own change, if you've made the requested trivial fix.

[joshtriplett]

@bors r+

[bors]

📌 Commit 68b0d86 has been approved by joshtriplett