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/include/mbedtls/config.h b/include/mbedtls/config.h
index 22d465c..69d2b63 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -269,7 +269,7 @@
  *            digests and ciphers instead.
  *
  */
-//#define MBEDTLS_AEAD_CHACHA20_POLY1305_ALT
+//#define MBEDTLS_CHACHAPOLY_ALT
 //#define MBEDTLS_AES_ALT
 //#define MBEDTLS_ARC4_ALT
 //#define MBEDTLS_BLOWFISH_ALT
@@ -1690,15 +1690,15 @@
 #define MBEDTLS_AES_C
 
 /**
- * \def MBEDTLS_AEAD_CHACHA20_POLY1305_C
+ * \def MBEDTLS_CHACHAPOLY_C
  *
  * Enable the ChaCha20-Poly1305 AEAD algorithm.
  *
- * Module:  library/aead_chacha20_poly1305.c
+ * Module:  library/chachapoly.c
  *
  * This module requires: MBEDTLS_CHACHA20_C, MBEDTLS_POLY1305_C
  */
-#define MBEDTLS_AEAD_CHACHA20_POLY1305_C
+#define MBEDTLS_CHACHAPOLY_C
 
 /**
  * \def MBEDTLS_ARC4_C
@@ -1855,7 +1855,7 @@
  * Enable the ChaCha20 block cipher.
  *
  * Module:  library/chacha20.c
- * Caller:  library/aead_chacha20_poly1305.c
+ * Caller:  library/chachapoly.c
  */
 #define MBEDTLS_CHACHA20_C
 
@@ -2427,7 +2427,7 @@
  * Enable the Poly1305 MAC algorithm.
  *
  * Module:  library/poly1305.c
- * Caller:  library/aead_chacha20_poly1305.c
+ * Caller:  library/chachapoly.c
  */
 #define MBEDTLS_POLY1305_C