Fix box_seal/2 and box_seal_open/3.

* Call the functions `box_seal` and `box_seal_open` to match the libsodium names in module `enacl`.
* Fix a bug in the C NIF: We should fail if the input is `<` SEALBYTES but not on `<=` SEALBYTES. The latter made it impossible to encode empty messages.
* Add variants which run directly on the interpreter scheduler for small messages.

Also:

* Provide full EQC functions for the testing purposes. This generated around 13000 random test cases in a 5 minute run, all passing.# Please enter the commit message for your changes. Lines starting