scx_layered: Fix core selection

[hodgesds]

Fix a bug introduced in #510 where it assumed core ids are incremental. This refactors the core ordering for layers to be far more simple and provide some space for layer core isolation in low utilization.

Tested by adding some logs on the core selection values:

$ sudo ./bin_local/bin/scx_layered --monitor 5 f:user.json -vvv 2>&1 | grep xxx
02:25:45 [TRACE] xxx layer random cores 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 
02:25:45 [TRACE] xxx layer hodgesd cores 36, 37, 38, 39, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 
02:25:45 [TRACE] xxx layer stress-ng cores 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 0, 1, 2, 3, 4, 5, 6, 7, 
02:25:45 [TRACE] xxx layer normal cores 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 

Using the following config:

[{
  "name":"random",
  "comment":"random user",
  "matches":[
     [{"UIDEquals":1234}]
  ],
  "kind": {
    "Confined":{
      "nodes":[1],
      "util_range": [0.05, 0.7],
      "preempt": false,
      "exclusive": true
     }
  }
},
{
  "name":"hodgesd",
  "comment":"hodgesd user",
  "matches":[
     [{"UIDEquals":224791}]
  ],
  "kind": {
    "Confined":{
      "nodes":[1],
      "util_range": [0.05, 0.3],
      "exclusive": false
     }
  }
},{
  "name":"stress-ng",
  "comment":"stress-ng slice",
  "matches":[
     [
             {"CommPrefix":"stress-ng"}
     ],[
             {"PcommPrefix":"stress-ng"}
     ]],
  "kind": {
    "Confined":{
      "util_range": [0.05, 0.70],
      "nodes":[1],
      "exclusive": false
     }
  }
},
{
  "name":"normal",
  "comment":"the rest",
  "matches":[[]],
  "kind":{
    "Grouped": {
      "util_range": [0.05, 0.7],
      "preempt": true,
      "exclusive": true,
      "nodes":[0]
    }
  }
}]

stress-ng run as user:

stress-ng run as root: