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Motivation is similar to NO_UDBL_DIVISION.

The alternative implementation of 64-bit mult is straightforward and aims at
obvious correctness. Also, visual examination of the generate assembly show
that it's quite efficient with clang, armcc5 and arm-clang. However current
GCC generates fairly inefficient code for it.

I tried to rework the code in order to make GCC generate more efficient code.
Unfortunately the only way to do that is to get rid of 64-bit add and handle
the carry manually, but this causes other compilers to generate less efficient
code with branches, which is not acceptable from a side-channel point of view.

So let's keep the obvious code that works for most compilers and hope future
versions of GCC learn to manage registers in a sensible way in that context.

See https://bugs.launchpad.net/gcc-arm-embedded/+bug/1775263

- fix some whitespace
- fix most overlong lines
- remove some superfluous parentheses
- s/result/ret/ for consistency with the rest of the library

- prefix is no necessary for static ids and makes lines longer
- most often omitted (even though we're not fully consistent)

This reduces clutter, making the functions more readable.

Also, it makes lcov see each line as covered. This is not cheating, as the
lines that were previously seen as not covered are not supposed to be reached
anyway (failing branches of the selftests).

Thanks to this and previous test suite enhancements, lcov now sees chacha20.c
and poly1305.c at 100% line coverage, and for chachapoly.c only two lines are
not covered (error returns from lower-level module that should never happen
except perhaps if an alternative implementation returns an unexpected error).

Also fix two validation bugs found while adding the tests.

Also handle test dependencies the right way while at it.

This module used (len, pointer) while (pointer, len) is more common in the
rest of the library, in particular it's what's used in the CMAC API that is
very comparable to Poly1305, so switch to (pointer, len) for consistency.

For consistency with the existing CMAC and HMAC APIs

This is a C99 feature and unfortunately we can't rely on it yet considering
the set of toolchain (versions) we want to support.

- in .h files: only put the context declaration inside the #ifdef _ALT
  (this was changed in 2.9.0, ie after the original PR)
- in .c file: only leave selftest out of _ALT: even though some function are
  trivial to build from other parts, alt implementors might want to go another
way about them (for efficiency or other reasons)

This change corrects some minor style violations, mostly for spacing
around parentheses.

This change permits users of the ChaCha20/Poly1305 algorithms
(and the AEAD construction thereof) to pass NULL pointers for
data that they do not need, and avoids the need to provide a valid
buffer for data that is not used.

Test vectors are included from RFC 7539.

Poly1305 is also added to the benchmark program.