doc: make the sentence make more sense
Guide: generics
Allow generic foreign functions.

Generic extern functions written in Rust have their names mangled, as well as their internal clownshoe __rust_abi functions. This allows e.g. specific monomorphizations of these functions to be used as callbacks.

Closes #12502.
Add new tests for extern and foreign fns and name mangling.
Fix typo in the tutorial
auto merge of #16220 : tshepang/rust/temp, r=steveklabnik
Temporary bootstrapping hack: introduce syntax for r egion bounds like `'b:'a`,
meaning `'b outlives 'a`. Syntax currently does nothing but is needed for full
fix to #5763. To use this syntax, the issue_5763_bootstrap feature guard is
required.
auto merge of #16318 : nikomatsakis/rust/issue-5723-bootstrap-2, r=pnkfelix

Introduce syntax for lifetime bounds like `'b:'a`, meaning `'b outlives 'a`. Syntax currently does nothing but is needed for full fix to #5763. To use this syntax, the issue_5763_bootstrap feature guard is required.
auto merge of #15831 : rpjohnst/rust/generic-foreign-fns, r=alexcrichton

This allows for things like this:

    extern "C" fn callback<T>(t: T) { /* ... */ }
    extern "C" {
        fn take_callback(c: extern fn(i32));
    }

and later:

    take_callback(callback::<i32>);

Closes #12502.
Guide: closures
auto merge of #16317 : adrienbrault/rust/patch-1, r=kballard
Remove spawn-stack-too-big.rs, which is too flaky.
auto merge of #16334 : nikomatsakis/rust/mission-snapshot, r=achrichto
Guide: Traits
auto merge of #16206 : steveklabnik/rust/guide_lambdas, r=brson
auto merge of #16273 : steveklabnik/rust/guide_generics, r=brson