Matcher tweaks for RE2.
Provide IRC metadata for all command expansion.
Rename Henzell -> Sequell.
Use RE2 for !learn edit
WIP new Sequell functions.
Fix numbers being forced to strings in let bindings.
More template expansion fixes.
Revert "Toss $ from sigils."

This reverts commit 7c114f89b78e75e25f98c7b75d06e87e8a8710a4.
(scope) and (binding).
Make (let) and (fn) less idiosyncratic.
Fix definition of set!
Don't post-expand do {} output in LearnDB queries.
Don't conflate PM with proxy / subcommands.
Add utc function.
Fix XSS in LearnDB html (rwbarton).
re match functions, better error reporting.
LearnDB access functions.
Document re functions, use a saner model for re-replace.
Report subcommand errors correctly.
Clean up tests and documentation.