As suggested in issue bitcoin-core#1381, this will make things simpler and
improve code readability, as we don't need to force omitting of
evaluations on a case-by-case basis anymore and hence can remove
lots of `#ifdef VERIFY`/`#endif` lines (see next commit). Plus,
VERIFY_CHECK behaves now identical in both non-VERIFY and coverage mode,
making the latter not special anymore and hopefully decreasing
maintenance burden. The idea of "side-effect safety" is given up.

Note that at two places in the ellswift module void-casts of return
values have to be inserted for non-VERIFY builds, in order to avoid
   "variable ... set but not used [-Wunused-but-set-variable]"
warnings.

Now that the `VERIFY_CHECK` compiles to empty in non-VERIFY mode, blocks
that only consist of these macros don't need surrounding `#ifdef VERIFY`
conditions anymore.

At some places intentional blank lines are inserted for grouping and
better readadbility.

By providing an uppercase variant of these verification functions, it is
better visible that it is test code and surrounding `#ifdef VERIFY`
blocks can be removed (if there is no other code around that could
remain in production mode), as they don't serve their purpose any more.

At some places intentional blank lines are inserted for grouping and
better readadbility.

By providing an uppercase variant of these verification functions,
it is better visible that it is test code.

As the fields r->x and r->y are set immediately after (three lines
below), there is no need to clear them.

This define was seemingly introduced for VERIFY mode code with side
effects (for setup purposes), that should just be executed without any
checks. The same can be achieved by putting it in an `#if VERIFY` block,
so we can remove it.