Optimize output of non-required properties

For this schema:

```
{
  "properties": {
    "a": {
      "const": "a"
    },
    "b": {
      "const": "b"
    },
    "c": {
      "const": "c"
    }
  }
}
```

We currently produce the following regex (spacing added around
alternatives for clarity)

```
\{("a":"a"(,"b":"b")?(,"c":"c")?
 |("a":"a",)?"b":"b"(,"c":"c")?
 |("a":"a",)?("b":"b",)?"c":"c")?\}
```

This works perfectly well, but contains redundancy. This is seen by the
fact that all three alternatives would match JSON with all three fields.

The difference between cases at the moment, is which field is mandatory.

I propose that we make the alternatives model the choice of last field.
This will produce a regex like this:

```
\{("a":"a"
 |("a":"a",)?"b":"b"
 |("a":"a",)?("b":"b",)?"c":"c")?\}
```

This will give us a shorter, but 100% equivalent regex.

src/json_schema/mod.rs

@@ -838,7 +838,7 @@ mod tests {
                     "title": "Character",
                     "type": "object"
                 }"#,
-                format!(r#"\{{([ ]?"name"[ ]?:[ ]?({STRING}|null)([ ]?,[ ]?"age"[ ]?:[ ]?({INTEGER}|null))?([ ]?,[ ]?"strength"[ ]?:[ ]?({INTEGER}|null))?|([ ]?"name"[ ]?:[ ]?({STRING}|null)[ ]?,)?[ ]?"age"[ ]?:[ ]?({INTEGER}|null)([ ]?,[ ]?"strength"[ ]?:[ ]?({INTEGER}|null))?|([ ]?"name"[ ]?:[ ]?({STRING}|null)[ ]?,)?([ ]?"age"[ ]?:[ ]?({INTEGER}|null)[ ]?,)?[ ]?"strength"[ ]?:[ ]?({INTEGER}|null))?[ ]?\}}"#).as_str(),
+                format!(r#"\{{([ ]?"name"[ ]?:[ ]?({STRING}|null)|([ ]?"name"[ ]?:[ ]?({STRING}|null)[ ]?,)?[ ]?"age"[ ]?:[ ]?({INTEGER}|null)|([ ]?"name"[ ]?:[ ]?({STRING}|null)[ ]?,)?([ ]?"age"[ ]?:[ ]?({INTEGER}|null)[ ]?,)?[ ]?"strength"[ ]?:[ ]?({INTEGER}|null))?[ ]?\}}"#).as_str(),
                 vec![
                     r#"{ "name" : "Player" }"#,
                     r#"{ "name" : "Player", "age" : 10, "strength" : 10 }"#,
@@ -898,7 +898,7 @@ mod tests {
         ] {
             let json: Value = serde_json::from_str(schema).expect("Can't parse json");
             let result = to_regex(&json, None).expect("To regex failed");
-            assert_eq!(result, regex);
+            assert_eq!(result, regex, "JSON Schema {} didn't match", schema);
 
             let re = Regex::new(&result).expect("Regex failed");
             for m in a_match {
@@ -1075,7 +1075,7 @@ mod tests {
         assert!(result.is_ok(), "{:?}", result);
         let regex = result.unwrap();
         assert_eq!(
-            r#"\{([ ]?"node"[ ]?:[ ]?\{([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*)([ ]?,[ ]?"next"[ ]?:[ ]?\{([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*))?[ ]?\})?|([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*)[ ]?,)?[ ]?"next"[ ]?:[ ]?\{([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*))?[ ]?\})?[ ]?\})?[ ]?\}"#,
+            r#"\{([ ]?"node"[ ]?:[ ]?\{([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*)|([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*)[ ]?,)?[ ]?"next"[ ]?:[ ]?\{([ ]?"value"[ ]?:[ ]?(-)?(0|[1-9][0-9]*))?[ ]?\})?[ ]?\})?[ ]?\}"#,
             regex,
         );
     }

src/json_schema/parsing.rs

@@ -161,9 +161,6 @@ impl<'a> Parser<'a> {
                     pattern += &format!("({}{},)?", subregex, self.whitespace_pattern);
                 }
                 pattern += &property_subregexes[i];
-                for subregex in &property_subregexes[i + 1..] {
-                    pattern += &format!("({},{})?", self.whitespace_pattern, subregex);
-                }
                 possible_patterns.push(pattern);
             }