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/library/version_features.c b/library/version_features.c
index b73410c..cce1a38 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -84,9 +84,9 @@
 #if defined(MBEDTLS_TIMING_ALT)
     "MBEDTLS_TIMING_ALT",
 #endif /* MBEDTLS_TIMING_ALT */
-#if defined(MBEDTLS_AEAD_CHACHA20_POLY1305_ALT)
-    "MBEDTLS_AEAD_CHACHA20_POLY1305_ALT",
-#endif /* MBEDTLS_AEAD_CHACHA20_POLY1305_ALT */
+#if defined(MBEDTLS_CHACHAPOLY_ALT)
+    "MBEDTLS_CHACHAPOLY_ALT",
+#endif /* MBEDTLS_CHACHAPOLY_ALT */
 #if defined(MBEDTLS_AES_ALT)
     "MBEDTLS_AES_ALT",
 #endif /* MBEDTLS_AES_ALT */
@@ -519,9 +519,9 @@
 #if defined(MBEDTLS_AES_C)
     "MBEDTLS_AES_C",
 #endif /* MBEDTLS_AES_C */
-#if defined(MBEDTLS_AEAD_CHACHA20_POLY1305_C)
-    "MBEDTLS_AEAD_CHACHA20_POLY1305_C",
-#endif /* MBEDTLS_AEAD_CHACHA20_POLY1305_C */
+#if defined(MBEDTLS_CHACHAPOLY_C)
+    "MBEDTLS_CHACHAPOLY_C",
+#endif /* MBEDTLS_CHACHAPOLY_C */
 #if defined(MBEDTLS_ARC4_C)
     "MBEDTLS_ARC4_C",
 #endif /* MBEDTLS_ARC4_C */