Examples: validation form using free applicative

This is a simple example of free applicative usage. `help` and `count` are two "static analysis" functions that can extract information from `Form a` without executing it.

Sample run:

```
Creating a new user.
Please, fill the form:
[1/3] Username: bob
error: the value is not available
[1/3] Username: bobby
[2/3] Full name: Bob White
[3/3] Age:
help: an integer value
[3/3] Age: 28
Successfully created user "bobby"!
```