AES LightEngine GCM Mode

[officialfrancismendoza]

Building upon the aes-lowmemory crate and associated work done in (#98, #99) as well as the new symmetric cipher traits and AEADCipherEncryptors and AEADCipherDecryptors done in (#119, #120), build AES lowmemory GCM mode.

[ounsworth]

Naming question: What is a "LightEngine"?

[dghgit]

In Bouncy Castle Java and C# these are basically the equivalent to -lowmemory in Rust - a trade of in performance for size. The original Java source (not that you could tell now) for what is in what is now called the aes crate, was the AESLightEgine class.