libsodium: update to 1.0.15

Version 1.0.15
 - The default password hashing algorithm is now Argon2id. The
`pwhash_str_verify()` function can still verify Argon2i hashes
without any changes, and `pwhash()` can still compute Argon2i hashes
as well.
 - The aes128ctr primitive was removed. It was slow, non-standard, not
authenticated, and didn't seem to be used by any opensource project.
 - Argon2id required at least 3 passes like Argon2i, despite a minimum
of `1` as defined by the `OPSLIMIT_MIN` constant. This has been fixed.
 - The secretstream construction was slightly changed to be consistent
with forthcoming variants.
 - The Javascript and Webassembly versions have been merged, and the
module now returns a `.ready` promise that will resolve after the
Webassembly code is loaded and compiled.
 - Note that due to these incompatible changes, the library version
major was bumped up.


(adam)