remove sodium-universal as dependency

sodium-universal is quite a heavy dependency, it ships sodium-native which has native C code, and overall a bunch of tricky deps like blake2b-wasm.

Turns out we're using it in ssb-keyring ONLY for getting constants, like the number 32 for the size of the buffers.

This PR will benefit Manyverse and other apps, by not requiring C compilation of the underlying libraries, and reducing the size of the node_modules folder.

This PR also adds tests that use na.crypto_secretbox_KEYBYTES to make sure that we are using the correct key sizes, but not at runtime.