Rewrite 'keys-as-symbols' macro for readability

Rather than nesting a bunch of calls, we'll thread the map's
keys through each transformation layer to make it more immediately
apparent that the macro's goal is to run a series of transforms
on each of the keys.
Document 'let-all' and 'keys-as-symbols' macros
Quickfix: use correct threading operator

I was using the '->' (thread as first argument) operator,
but collection operations are supposed to use the '->>'
(thread as last argument) operator.