Auto merge of #2315 - saethlin:shrink-item, r=saethlin

Optimizing Stacked Borrows (part 2): Shrink Item

This moves protectors out of `Item`, storing them both in a global `HashSet` which contains all currently-protected tags as well as a `Vec<SbTag>` on each `Frame` so that when we return from a function we know which tags to remove from the protected set.

This also bit-packs the 64-bit tag and the 2-bit permission together when they are stored in memory. This means we theoretically run out of tags sooner, but I doubt that limit will ever be hit.

Together these optimizations reduce the memory footprint of Miri when executing programs which stress Stacked Borrows by ~66%. For example, running a test with isolation off which only panics currently peaks at ~19 GB, with this PR it peaks at ~6.2 GB.

To-do
- [x] Enforce the 62-bit limit
- [x] Decide if there is a better order to pack the tag and permission in
- [x] Wait for `UnsafeCell` to become infectious, or express offsets + tags in the global protector set

Benchmarks before:
```
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/backtraces/Cargo.toml
  Time (mean ± σ):      8.948 s ±  0.253 s    [User: 8.752 s, System: 0.158 s]
  Range (min … max):    8.619 s …  9.279 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/mse/Cargo.toml
  Time (mean ± σ):      2.129 s ±  0.037 s    [User: 1.849 s, System: 0.248 s]
  Range (min … max):    2.086 s …  2.176 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
  Time (mean ± σ):      3.334 s ±  0.017 s    [User: 3.211 s, System: 0.103 s]
  Range (min … max):    3.315 s …  3.352 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde2/Cargo.toml
  Time (mean ± σ):      3.316 s ±  0.038 s    [User: 3.207 s, System: 0.095 s]
  Range (min … max):    3.282 s …  3.375 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
  Time (mean ± σ):      6.391 s ±  0.323 s    [User: 5.928 s, System: 0.412 s]
  Range (min … max):    6.090 s …  6.917 s    5 runs
 ```
 After:
 ```
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/backtraces/Cargo.toml
  Time (mean ± σ):      6.955 s ±  0.051 s    [User: 6.807 s, System: 0.132 s]
  Range (min … max):    6.900 s …  7.038 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/mse/Cargo.toml
  Time (mean ± σ):      1.784 s ±  0.012 s    [User: 1.627 s, System: 0.156 s]
  Range (min … max):    1.772 s …  1.797 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
  Time (mean ± σ):      2.505 s ±  0.095 s    [User: 2.311 s, System: 0.096 s]
  Range (min … max):    2.405 s …  2.603 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde2/Cargo.toml
  Time (mean ± σ):      2.449 s ±  0.031 s    [User: 2.306 s, System: 0.100 s]
  Range (min … max):    2.395 s …  2.467 s    5 runs

Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
  Time (mean ± σ):      3.667 s ±  0.110 s    [User: 3.498 s, System: 0.140 s]
  Range (min … max):    3.564 s …  3.814 s    5 runs
 ```
 The decrease in system time is probably due to spending less time in the page fault handler.

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