Update crypto/aead to libsodium 1.0.15

[silkeh]

For performing the size checks from libsodium 1.0.14 the support.CheckSizeMax function is added.

libsodium also added a warning to crypto_aead_aes256gcm (below), should we include it as well?

/*
 * WARNING: Despite being the most popular AEAD construction due to its
 * use in TLS, safely using AES-GCM in a different context is tricky.
 *
 * No more than ~ 350 GB of input data should be encrypted with a given key.
 * This is for ~ 16 KB messages -- Actual figures vary according to
 * message sizes.
 *
 * In addition, nonces are short and repeated nonces would totally destroy
 * the security of this scheme.
 *
 * Nonces should thus come from atomic counters, which can be difficult to
 * set up in a distributed environment.
 *
 * Unless you absolutely need AES-GCM, use crypto_aead_xchacha20poly1305_ietf_*()
 * instead. It doesn't have any of these limitations.
 * Or, if you don't need to authenticate additional data, just stick to
 * crypto_secretbox().
 */