I have a usecase where specific nice values are used to bucket tasks
into groups that are handled separately by different `scx_layered`
policies, with no implications of relative priority between niceness X,
X + 1, X - 1, etc. In other words, nicevals are used as simple tags in
this scenario.

If we wanted to treat a specific niceness this way e.g. `11`, we could
do so with AND'd MATCH_NICE_{ABOVE,BELOW} like so:

```json
  "matches" : [
    [
      {
        "NiceAbove": 10
      },
      {
        "NiceBelow": 12
      },
    ],
  ],
```

But this is unnecessarily verbose and doesn't communicate the intent of
the match very well. Adding a `NiceEquals` matcher simplifies the
config and makes intent obvious:

```json
  "matches" : [
    [
      {
        "NiceEquals": 11
      },
    ],
  ],
```

This PR adds support for such a matcher.

Also, rename `layer_match.nice_above_or_below` to just
`layer_match.nice`, as the former doesn't describe the newly-added
matcher's use of the field. It's still obvious that `layer_match.nice`
is relevant to MATCH_NICE_{ABOVE, BELOW, EQUALS}.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>