121 replace assert with check

[milancurcic]

This PR implements the check subroutine, proposed to replace assert for internal testing. Specifically:

• Implements check as proposed here
• Replaces existing calls to assert with calls to check
• Removes the definition of assert from stdlib_experimental_error

This PR doesn't cover making all check calls to print meaningful messages on failure, and to execute them with warn=.true., to allow multiple test failures in a single run. These will be covered in a future PR.

This internal testing is meant as temporary until we can do it with fpm test as proposed in #162. For now, we consider this as a "good enough" solution that will help us move forward with development while fpm is in the works.

Finally, there is also Brad's proposed implementation here which we shouldn't ignore. If there is some support from the community for it, let's discuss and consider it.

[certik]

That looks good.

[ivan-pi]

I left only two minor comments.

[ivan-pi]

Wouldn't it make more sense if the default message is `'Check failed.' just to be consistent with the subroutine name?

[ivan-pi]

There is an implicit assumption in the examples that a is not five. It remains to be discussed whether such examples in the docstrings should be self-contained.

[milancurcic]

@ivan-pi You're right. The latest commit fixes that.

[certik]

That looks good to me.

[jvdp1]

It looks good to me. Thank you.

[everythingfunctional]

I'm ok with it. I think we should still come up with another function that behaves like I suggested. Obviously we'll have to come up with a new name though.

[milancurcic]

Thanks, all. @everythingfunctional, let's do a separate issue and PR for your proposal when you're ready. Merging.

[jvdp1]

@milancurcic Do we want that users use both check and error_stop outside stdlib? OR are these only aimed for stdlib?
If users can use them outside stdlib, then I think it would be good to have some specs (like for other functions) (I can give a start based on the comments in the code). Personally, I could use both outside stdlib ;)

[milancurcic]

@jvdp1 I think so. Sorry, I totally forgot about the spec. That should've been part of the original PR. Please go ahead with a draft spec if you don't mind. Thank you!