Rethink lib/validator top level subdirs

[waalge]

(Close if this already exists - couldn't see it)

This design seems arbitrary and undocumented.
That you can, say, use my_other_validator but only from a module in ./validators isnt (afaics) documented.
You also have namespace pollution:

$tree .
.
├── aiken.lock
├── aiken.toml
├── build ... 
├── lib
│   ├── example
│   └── example.ak
├── README.md
└── validators
    └── example.ak 

breaks

My current desire is test validators.
It is non-obvious that

validator {
  pub fn my_validator(_, _) { }
}

is how to make validators publicly available, as if it were a top level function.

It would be pleasant to have a top level ./tests which can use from anywhere