We have changed too much to consider it a mere bundled dependency (such
as unicode handling in e3bfcf2fd4a4ebf00b104b082cfe83c8144a842d), and
can consider it our own at this point.

It's probably not worth adding the C test files to regular formatting as
they're pretty much never touched, but ensuring the files are formatted
according to our standards and getting rid of warnings is a cheap
one-time fix.

In order to run unittests with a release build, we need the test
functions to be accessible when NDEBUG is defined. Moving the functions
into the test fixture ensures they are available and only available for
use by the unit tests.

If you like it you shouldn't put a ring on it.

This is what _every_ consumer of RStream used anyway, either by calling
rbuffer_reset, or rbuffer_consumed_compact (same as rbuffer_reset
without needing a scratch buffer), or by consuming everything in
each stream_read_cb call directly.

A varargs functions can never be inlined, so a macro is faster.

We already have an extensive suite of static analysis tools we use,
which causes a fair bit of redundancy as we get duplicate warnings. PVS
is also prone to give false warnings which creates a lot of work to
identify and disable.

libnvim couldn't be easily used in C++ due to the use of reserved keywords.

Additionally, add explicit casts to *alloc function calls used in inline
functions, as C++ doesn't allow implicit casts from void pointers.