Migration to null safety

[cryoelite]

An attempt to migrate to null safety, I have made minor changes and with them currently

1. Package is able to be compiled in null-safe projects.
2. File Decryption works just like before.
3. String Encryption works just like before.

I have not tested out other functions and features, nor have I checked the code itself for bugs. So I would request proper verification and modification before accepting the pull. The reason I have created this PR is to let everyone know that this version can successfully do the above tasks and hence be used with null-safe projects if needed.

[cryoelite]

If someone wants to include my package as-is right now, that is from my github then simply add:

aes_crypt: git: url: https://github.com/cryoelite/aes_crypt.git ref: master

under dev_dependencies in pubspec.yaml of your project. (with proper indentation)