Formalize user documentation

[charleskawczynski]

This package will likely soon become quite pervasive throughout our ecosystem, and we should have good documentation for:

• What this package does
• Usage
• Benefits
• Caveats
• How it works
• Background information
• Motivation

I've added a bunch of documentation already in hopes to check off each of these boxes, but users will ultimately judge the quality of the documentation, and what pieces need improvement.

[charleskawczynski]

Thoughts, @Sbozzolo?

[charleskawczynski]

@dennisYatunin suggested adding downsides:

• Compile times
• Inference?
• Stack pressure (lines of code-typed / llvm / native, unique ssa slots)