Document liballoc::rc

This commit introduces a bunch of documentation, fixes some consistency
issues, and just basically brings Rc<T> and Weak<T> up to snuff.
rustdoc: inherited vis. for struct variant fields

Teach rustdoc that struct variant fields have inherited visibility.

Fix #19048
Documentation fix for std::num::strconv

- int_to_str_bytes_common() doesn't have a return value.
- float_to_str_bytes_common() has an old-style doc comment.
Fixed iOS build after Iter stab
Fix example code for unreachable!

The previous code was giving an incorrect result (not x/3).
debuginfo: Make variables captured in unboxed closures available in debuginfo.
auto merge of #19112 : steveklabnik/rust/doc_rc, r=Gankro
auto merge of #19349 : tomjakubowski/rust/rustdoc-struct-variant-vis, r=sfackler

Teach rustdoc that struct variant fields have inherited visibility.

Fix #19048
auto merge of #19354 : barosl/rust/strconv-doc-fix, r=steveklabnik

- `int_to_str_bytes_common()` doesn't have a return value.
- `float_to_str_bytes_common()` has an old-style doc comment.
auto merge of #19355 : vhbit/rust/ios-backtrace-fix, r=alexcrichton
auto merge of #19360 : olivren/rust/master, r=Gankro

The previous code was giving an incorrect result (not x/3).

Also, this function does not work with signed integers. It now accepts `u32` instead of `i32`.
auto merge of #19363 : michaelwoerister/rust/support-unboxed-closures, r=alexcrichton

This PR lets `rustc` generate debuginfo for variables captured by unboxed closures. 

Fixes #19356

@nikomatsakis This PR will probably conflict with #19338. If this gets merged before, you should be able to just leave the test case as it is (maybe remove the `#![feature(unboxed_closures)]` directive).