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.
In addition to making the APIs of the various AEAD modules more consistent
with each other, it's useful to have an auth_decrypt() function so that we can
safely check the tag ourselves, as the user might otherwise do it in an
insecure way (or even forget to do it altogether).
While the old name is explicit and aligned with the RFC, it's also very long,
so with the mbedtls_ prefix prepended we get a 31-char prefix to each
identifier, which quickly conflicts with our 80-column policy.
The new name is shorter, it's what a lot of people use when speaking about
that construction anyway, and hopefully should not introduce confusion at
it seems unlikely that variants other than 20/1305 be standardised in the
foreseeable future.
