Easycheck

This release comes with type hints for `easycheck` functions.

This is a big release, with two important functionalities added:
* `easycheck` functions can be switched off using an environmental variable
* assertion functions work just like assertions, so when `__debug__` is `True`

Additional functionalities are related to the repository and documentation:
* badges were added
* CI was set up in GitHub actions

This release comes with a significant performance improvement. Previously, `easycheck` functions ran various validations of their arguments, and used `check_if()` and some other `easycheck` functions to run checks. Many of the validations are removed, as Python runs them anyway. All `check_if()` checks were replaced with the corresponding `if`-based checks. That way, `easycheck` is much faster, sometimes almost up to 100 times.

This is not the end of the optimizations. In future releases, the `_raise` function, used by all `easycheck` functions, will be improved in terms of performance. Maybe all instances of `easycheck` functions run by other `easycheck` functions will be replaced with the corresponding `if`-checks, but this may lead to a far more complicated code.

The release comes with a new function, `check_if_in_limits`, and its testing alias, `assert_if_in_limits`.

This release comes with improved messaging. For custom exception classes, if a message is not provided, the docstring of exception class is taken as the message (this is the default). For built-in exceptions, the default behavior is to not print any message.

Now you can check an object's type with a `check_type()` function. For backward compatibility, the previous name, `check_instance()`, is kept, and will be kept for some time.