Rename aead_chacha20_poly1305 to chachapoly

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.

library/CMakeLists.txt

@@ -3,7 +3,6 @@ option(USE_SHARED_MBEDTLS_LIBRARY "Build mbed TLS shared library." OFF)
 option(LINK_WITH_PTHREAD "Explicitly link mbed TLS library to pthread." OFF)
 
 set(src_crypto
-    aead_chacha20_poly1305.c
     aes.c
     aesni.c
     arc4.c
@@ -15,6 +14,7 @@ set(src_crypto
     camellia.c
     ccm.c
     chacha20.c
+    chachapoly.c
     cipher.c
     cipher_wrap.c
     cmac.c