Remove use of Option<T> inside of RingBuf

Fix for task in Metabug #18009

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.

## Benchmark
**Before:**
```
test ringbuf::tests::bench_grow_1025                       ... bench:      9200 ns/iter (+/- 100)
test ringbuf::tests::bench_iter_1000                       ... bench:      1341 ns/iter (+/- 34)
test ringbuf::tests::bench_mut_iter_1000                   ... bench:      2250 ns/iter (+/- 44)
test ringbuf::tests::bench_new                             ... bench:        22 ns/iter (+/- 0)
test ringbuf::tests::bench_pop_100                         ... bench:       351 ns/iter (+/- 3)
test ringbuf::tests::bench_pop_front_100                   ... bench:      1169 ns/iter (+/- 61)
test ringbuf::tests::bench_push_back_100                   ... bench:       244 ns/iter (+/- 3)
test ringbuf::tests::bench_push_front_100                  ... bench:       246 ns/iter (+/- 5)
```
**After:**
```
test ringbuf::tests::bench_grow_1025                       ... bench:      4005 ns/iter (+/- 96)
test ringbuf::tests::bench_iter_1000                       ... bench:       657 ns/iter (+/- 7)
test ringbuf::tests::bench_mut_iter_1000                   ... bench:       633 ns/iter (+/- 7)
test ringbuf::tests::bench_new                             ... bench:        15 ns/iter (+/- 0)
test ringbuf::tests::bench_pop_100                         ... bench:       210 ns/iter (+/- 0)
test ringbuf::tests::bench_pop_front_100                   ... bench:       179 ns/iter (+/- 1)
test ringbuf::tests::bench_push_back_100                   ... bench:       187 ns/iter (+/- 1)
test ringbuf::tests::bench_push_front_100                  ... bench:       174 ns/iter (+/- 2)
```
Theoretically, this could be

```rust
pub struct RingBuf<T> {
    head: uint,
    // len is the tail position
    data: Vec<T>,
}
```

which would make my comments below about using `Vec` automatic, but this may be worse due to being more verbose/less obvious. (The destructor would `.set_len(0)` at the end, to avoid the `Vec` destructor doing any element clean-up.)