Convert `principal-submitter` into a list

There are schemes, like SABER (#192) that have more than one principal
submitter. Consistency warrants that we turn it into a list for all
schemes and don't do something with allowing either a str or a list:
that would just be very annoying to parse.

Closes #194

Update OSX image

LEDAcrypt KEM-LT

Skip SPHINCS+ on Travis CI

We're running out of build limits.

Polish Dilithium

* fixes dynamic memory allocation test. previously a function called freeze() would trigger it

* this adds DilithiumII. Preprocessor conditionals still need to be removed

* fix ms Makefile

* fix MS compiler warnings

* clean-up

* remove preprocessor conditionals

* add dilithium3

* add dilithium4

* add duplicate consistency checks

* SHA2 state constants in common

* clean up symmetric.h

* Port SPHINCS+-SHA256 to sha256ctx struct

* Implement ctx struct for fips202

* Port Kyber{512,768,1024} to fips202 ctx struct

* Port NewHope to fips202 structs

* Port SPHINCS+-SHAKE256 to fips202 ctx structs

* Use opaque fips202 structs in MQDSS

* port dilithium to use fips202 ctx structs

* include -Wredundant-decls

* remove comment; format NTT constants

* reduce casts in power2round

update fork

Add cSHAKE{128,256} to common

Expand the pattern rule for the three common tests.

Fixed link to rust project in README.md

Fixes https://github.com/PQClean/PQClean/issues/178.