Expand the benchmarking and unit test suite for ring_buf.
-Adds unit tests for fn get() and fn get_mut() which are currently untested
-Adds unit tests to verify growth of the ringbuffer when reserve is called.
-Adds unit tests to confirm that dropping of items is correct
Move ringbuf to use a raw buffer instead of Option<T>
Handle allocate/reallocate errors in ring_buf
Use is_some() in clear to simplify the clear loop.
Added some extra debug_asserts to ring_buf.
Manually reset the ringbuffer before or after the ringbuffer push/pop tests.
Added population count assertion in reserve. Cleaned up wrap_index.
Update ring_buf.rs from fallout of #18827.
Un-feature gate struct variants

Struct variant field visibility is now inherited. Remove `pub` keywords
from declarations.

Closes #18641

[breaking-change]
doc: make end comment consistent with start comment
auto merge of #18994 : sfackler/rust/struct-variants-pt2, r=jakub-

Struct variant field visibility is now inherited. Remove `pub` keywords
from declarations.

Closes #18641

[breaking-change]

r? @alexcrichton
auto merge of #18995 : alfie/rust/comment-docs, r=aturon

Start comment is a string literal while end comment is made up of two character literals. This change is to make them consistent.
auto merge of #18747 : csherratt/rust/ringbuf-remove-option, r=huonw

Fix for task in Metabug #18009 (Rebased version of https://github.com/rust-lang/rust/pull/18170)

This changes much of about how RingBuf functions. `lo`, `nelts` are replaced by a more traditional `head` and`tail`. The `Vec<Option<T>>` is replaced by a bare pointer that is managed by the `RingBuf` itself. This also expects the ring buffer to always be size that is a power of 2.

This change also includes a number of new tests to cover the some areas that could be of concern with manual memory management.

The benchmarks have been reworked since the old ones were benchmarking of the Ring buffers growth rather then the actual test.

The unit test suite have been expanded, and exposed some bugs in `fn get()` and `fn get_mut()`

## Benchmark
**Before:**
```
test ring_buf::tests::bench_grow_1025                      ... bench:      8919 ns/iter (+/- 87)
test ring_buf::tests::bench_iter_1000                      ... bench:       924 ns/iter (+/- 28)
test ring_buf::tests::bench_mut_iter_1000                  ... bench:       918 ns/iter (+/- 6)
test ring_buf::tests::bench_new                            ... bench:        15 ns/iter (+/- 0)
test ring_buf::tests::bench_pop_100                        ... bench:       294 ns/iter (+/- 9)
test ring_buf::tests::bench_pop_front_100                  ... bench:       948 ns/iter (+/- 32)
test ring_buf::tests::bench_push_back_100                  ... bench:       291 ns/iter (+/- 16)
test ring_buf::tests::bench_push_front_100                 ... bench:       311 ns/iter (+/- 27
```
**After:**
```
test ring_buf::tests::bench_grow_1025                      ... bench:      2209 ns/iter (+/- 169)
test ring_buf::tests::bench_iter_1000                      ... bench:       534 ns/iter (+/- 27)
test ring_buf::tests::bench_mut_iter_1000                  ... bench:       515 ns/iter (+/- 28)
test ring_buf::tests::bench_new                            ... bench:        11 ns/iter (+/- 0)
test ring_buf::tests::bench_pop_100                        ... bench:       170 ns/iter (+/- 5)
test ring_buf::tests::bench_pop_front_100                  ... bench:       171 ns/iter (+/- 11)
test ring_buf::tests::bench_push_back_100                  ... bench:       172 ns/iter (+/- 13)
test ring_buf::tests::bench_push_front_100                 ... bench:       158 ns/iter (+/- 12)

```