Improved Compatibility and Easier Builds

This small release makes fscrypt much easier to build and use.

• PasswordHash has completely moved to x/crypto/argon2, eliminating the libargon2 build and runtime dependancy. Now the dependancies to build fscrypt are go, make, gcc, and some system headers. That's it! (#84)
• PasswordHash will only use at most 128MiB. This allows users to encrypt files on removable media and rest assured that it will still work when plugged into another system with less memory. (#74)
• fscrypt's build and CI systems have been greatly improved. All dependancies are now vendored with dep allowing for reproducible builds. Building, testing, and changing fscrypt is now much more straightforward. (#83, #85, #86, #88, #89, #90)
• Other minor fixes (#70, #75, #87)