`EraseIf` itself is not very hot function, but I want to use that as demonstration of the speed of iteration via `IterateOverFullSlots`.

Motivation:
1. We are still going to save some resources.
2. It is the first step to implement faster `absl::c_for_each` that may give larger benefits. Will require readability considerations.

`BM_EraseIf/num_elements:1000/num_erased:0` is just iteration and it gives 60% speed up.
On smaller tables  removing all elements shows 25% speed up. Note that on small tables erasing is much faster due to cl/592272653.

```
name                                          old cpu/op   new cpu/op   delta
BM_EraseIf/num_elements:10/num_erased:0       3.41ns ± 5%  3.03ns ± 3%  -11.14%  (p=0.000 n=37+35)
BM_EraseIf/num_elements:1000/num_erased:0     6.06ns ± 3%  2.42ns ± 3%  -60.05%  (p=0.000 n=34+37)
BM_EraseIf/num_elements:10/num_erased:5       5.90ns ± 3%  4.44ns ± 4%  -24.88%  (p=0.000 n=36+37)
BM_EraseIf/num_elements:1000/num_erased:500   11.0ns ± 2%   9.2ns ± 2%  -16.60%  (p=0.000 n=35+37)
BM_EraseIf/num_elements:10/num_erased:10      8.03ns ± 3%  5.77ns ± 2%  -28.19%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:1000  9.00ns ± 3%  7.83ns ± 2%  -12.98%  (p=0.000 n=37+37)

name                                          old time/op          new time/op          delta
BM_EraseIf/num_elements:10/num_erased:0       3.42ns ± 5%          3.04ns ± 3%  -11.13%  (p=0.000 n=37+36)
BM_EraseIf/num_elements:1000/num_erased:0     6.07ns ± 3%          2.42ns ± 3%  -60.10%  (p=0.000 n=34+37)
BM_EraseIf/num_elements:10/num_erased:5       5.93ns ± 3%          4.45ns ± 4%  -24.89%  (p=0.000 n=36+37)
BM_EraseIf/num_elements:1000/num_erased:500   11.1ns ± 2%           9.2ns ± 2%  -16.61%  (p=0.000 n=35+37)
BM_EraseIf/num_elements:10/num_erased:10      8.06ns ± 3%          5.79ns ± 2%  -28.19%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:1000  9.03ns ± 3%          7.85ns ± 2%  -12.98%  (p=0.000 n=37+37)

name                                          old INSTRUCTIONS/op  new INSTRUCTIONS/op  delta
BM_EraseIf/num_elements:10/num_erased:0         19.5 ± 1%            14.9 ± 0%  -23.79%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:0       19.9 ± 0%            12.7 ± 0%  -36.20%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:10/num_erased:5         36.9 ± 1%            30.9 ± 0%  -16.47%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:500     44.8 ± 0%            37.6 ± 0%  -16.06%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:10/num_erased:10        53.0 ± 1%            46.9 ± 0%  -11.61%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:1000    69.8 ± 0%            62.6 ± 0%  -10.32%  (p=0.000 n=36+37)

name                                          old CYCLES/op        new CYCLES/op        delta
BM_EraseIf/num_elements:10/num_erased:0         6.10 ± 7%            4.91 ± 1%  -19.49%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:0       19.4 ± 1%             7.7 ± 2%  -60.04%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:10/num_erased:5         13.9 ± 4%             9.2 ± 3%  -33.80%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:1000/num_erased:500     35.5 ± 0%            29.5 ± 1%  -16.74%  (p=0.000 n=37+37)
BM_EraseIf/num_elements:10/num_erased:10        20.8 ± 5%            13.5 ± 0%  -35.07%  (p=0.000 n=37+30)
BM_EraseIf/num_elements:1000/num_erased:1000    28.9 ± 0%            25.1 ± 0%  -13.06%  (p=0.000 n=37+37)
```

PiperOrigin-RevId: 642016364
Change-Id: I8be6af5916bd45fd110bb0398c3ffe932a6a083f