Migrate to re2

[PhilippMDoerner]

The regex lib appears to have deprecated its re/RegexMatch/Regex API in favour of something more macro based (?).
The API is identical, but the types and symbol-names are different.

You notice this if you use regex-routes in prologue, as you now get these warnings:
Warning: use re2(static string) instead; re is deprecated [Deprecated]

Now they're re2/RegexMatch2/Regex2. I did 5 minutes of playing around and it seems this will require adjusting a few things. The few I could think of:

• Adjust exports of core/basicregex.nim
• Adjust addRoute proc in core/applications.nim
• Adjust initRePath proc in core/route.nim

[nitely]

I think this can be done without breaking changes or deprecations. Regex2 is a distinct Regex, so I could un-deprecate the re API, and then we can just do Regex2(theRoute) internally, and use the new APIs. The old regex APIs will stay for a while (years), so there is no rush.