Direct linking guide

[danielcompton]

It could be good to have a guide on using direct linking which:

• Shows an example of how to set it up in a Leiningen and Boot application
• Shows in a REPL/code session how var indirection is removed with direct linking on
• Explains where you are likely to need ^:dynamic and ^:redef
• Shows an example of code that gets speed improvements from direct linking with benchmarks
• Which code direct linking applies to (just your code, or all your dependencies too?)
• How to distribute direct linked libraries
• Any tradeoffs or other reasons why you wouldn't want to use direct linking

[puredanger]

I have started working on this.