schema/v5.0: introduce computable version ranges

[rsc]

The shorthand version of this schema is:

defaultStatus: $status
versions: [{
    version: $version
    status: $status  // unknown, affected, unaffected

    versionType: string (‘semver’, ‘git’, ..., to define meaning of <)
    repo: string (optional, intended for versionType ‘git’)
    lessThan/lessThanOrEqual: $version (can use * for “infinity” aka "maxuint")
    changes: [{
        at: version where status changes
        status: ...
    }]
}]

An object in the versions list can be either:

• a simple {version: V, status: S},
  which indicates the status of the single version V.
• a range {version: V, versionType: T, lessThan: L OR lessThanOrEqual: LE, status: S, changes: C},
  which indicates the status of the half-open interval [V, L) or closed interval [V, LE].
  The range starts with V having status S and then changes over time according to the events listed in C.

The algorithm for deciding the status of a particular version V is then:

for entry in product.versions {
    if entry.lessThan is not present and entry.lessThanOrEqual is not present and v == entry.version {
        return entry.status
    }
    if (entry.lessThan is present and entry.version <= v and v < entry.lessThan) or
       (entry.lessThanOrEqual is present and entry.version <= v and v <= entry.lessThanOrEqual) {
        status = entry.status
        for change in entry.changes {
            if change.at <= v {
                status = change.status
            }
        }
        return status
    }
}
return product.defaultStatus

Fixes #87.
Fixes #12.
Fixes #77.

[rsc]

I've posted this as supporting material for my latest draft schema in #87 (comment). Discussion is probably better there than on this PR.

[rsc]

Made suggested changes. Not sure whether the other two additionalProperties: false should be kept or deleted.

[chandanbn]

I suggest deleting the other two additionalProperties: false as well.

[rsc]

Pushed a new commit that deletes the other two additionalProperties: false and also fixes an inverted comparison in the algorithm, pointed out by Oliver.

[rsc]

Based on Tuesday meeting discussion, pushed a new version and updated the comment at the top of this PR.
Will summarize the changes on #87.

[rsc]

Updated as requested. Also defined that default defaultStatus is unknown. And fixed bug where "limit" was still defined as the field instead of "lessThan" and "lessThanOrEqual", despite mentioning the new names everywhere else.

[rsc]

Changed to allow either defaultStatus or versions to be omitted, but not both.

[rsc]

Also updated to mention version 0 as the earliest possible start of a range.