Add a ``FLINT_ASSERT_ALWAYS`` macro

[fredrik-johansson]

Idea: have an assert statement that is executed regardless of --enable-assert.

Whereas FLINT_ASSERT can do checks that are expensive to compute and thus needs to be disabled by default, FLINT_ASSERT_ALWAYS would be used for strategically placed inexpensive checks, i.e. as a shorthand for the current idiom if (!cond) flint_throw(...).

This would also be useful during development as a way to test individual FLINT_ASSERT macros without having to reconfigure FLINT with --enable-assert, by temporarily changing them to FLINT_ASSERT_ALWAYS.

[albinahlback]

I'd rather name is something else, like CHECK, where the lack of prefix FLINT is due to it being internal only which saves some space on each line as well.

[edgarcosta]

You could also have asserts with different levels. ASSERT, ASSERT1, etc.. Ordered from cheap/important to expensive/stringent