Fixed up encoding issues

[josephlr]

I significantly constrained the space of admissible encodings to deal with #143 .

The keyid and data parameters in the encoding are gone, as they are either not used in validation, or they cause validation to fail. Similarly, the salt is now required. Before if no salt was provided, the encoding wouldn't include the salt or the hash value. Because there was no hash value, any later validation check would fail.

I also made changes to validate_context. Empty vs. NULL passwords and salts now produce the same results and errors. The salt was the real outlier here, a NULL salt was allowed while an empty salt produced an error (ARGON2_SALT_TOO_SHORT). This is inconsistent with the rest of Argon2, where switching a parameter from empty to NULL produces the exact same hash value (or produces the exact same error).

Note that this doesn't break virtually any consumers of the public API in argon2.h because all encodings emitted from functions like argon2_hash() are still valid. There are only two cases where an existing consumer would see breakage:

1. They had manually added a data parameter to the string encoding. Note that this parameter has always been ignored.
2. They were hashing with a NULL salt (because hashing with an empty salt doesn't work). This is quite insecure, and they already would have been unable to use argon2_verify() as it would just fail when decoding the encoded string.

I also added tests for a bad salt in an encoding string.

[codecov-io]

Current coverage is 67.47% (diff: 61.53%)

Merging #173 into master will increase coverage by 0.71%

@@             master       #173   diff @@
==========================================
  Files             9          9          
  Lines          1041       1033     -8   
  Methods         132        132          
  Messages          0          0          
  Branches        171        166     -5   
==========================================
+ Hits            695        697     +2   
+ Misses          266        261     -5   
+ Partials         80         75     -5   

Powered by Codecov. Last update 07524a3...5d4f755

[veorq]

Thanks!