=act reimplement actor path validation without regex

While working on fixing https://github.com/akka/akka/pull/16347 I decided to let go of the regexp and impl simpler methods.

I found that on a smaller scale the new methods are way better, but in the grand scheme of things there seems to be a larger trade off at higher percentiles.

I got to:

```
+[info] a.a.ActorCreationBenchmark.synchronousStarting       ss     20000       16.413        1.494       us
```

from an initial:

```
+[info] a.a.ActorCreationBenchmark.synchronousStarting       ss     20000       27.022        1.903       us
```

but the higher percentiles got hit quite hard (*averages mean nothing* mantra kicks in...).

Note: These implementations pass tests in akka actor and survive the `...|%67` which caused the regex based impl to blow up.

Do you think it is worth trying one of these impls or am I on the wrong way?
comparing the 3 variants I find this attractive (but with `charAt` and `tailrec`)
that should be all allocation free (as far as I can see)