=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?
Yes, I think that `charAt` will be inlined by the compiler in the end. I don't see that much difference between this one and `validElementHarder`.