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/error.h b/include/mbedtls/error.h
index 72b7f18..e056975 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -78,7 +78,7 @@
  * SHA512    1                  0x0039-0x0039
  * CHACHA20  1                  0x003B-0x003B
  * POLY1305  1                  0x0041-0x0041
- * AEAD_CHACHA20_POLY1305 2     0x0047-0x0049
+ * CHACHAPOLY 2                 0x0047-0x0049
  *
  * High-level module nr (3 bits - 0x0...-0x7...)
  * Name      ID  Nr of Errors