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.
diff --git a/programs/test/selftest.c b/programs/test/selftest.c
index 57f9924..13fa98c 100644
--- a/programs/test/selftest.c
+++ b/programs/test/selftest.c
@@ -46,7 +46,7 @@
 #include "mbedtls/camellia.h"
 #include "mbedtls/chacha20.h"
 #include "mbedtls/poly1305.h"
-#include "mbedtls/aead_chacha20_poly1305.h"
+#include "mbedtls/chachapoly.h"
 #include "mbedtls/base64.h"
 #include "mbedtls/bignum.h"
 #include "mbedtls/rsa.h"
@@ -216,8 +216,8 @@
 #if defined(MBEDTLS_POLY1305_C)
     {"poly1305", mbedtls_poly1305_self_test},
 #endif
-#if defined(MBEDTLS_AEAD_CHACHA20_POLY1305_C)
-    {"chacha20-poly1305", mbedtls_aead_chacha20_poly1305_self_test},
+#if defined(MBEDTLS_CHACHAPOLY_C)
+    {"chacha20-poly1305", mbedtls_chachapoly_self_test},
 #endif
 #if defined(MBEDTLS_BASE64_C)
     {"base64", mbedtls_base64_self_test},