Consider renaming `run!` macro

[Deraen]

This issue replaces an old PR: #150

The problem:

reagent.ratom/run! macro name conflicts with clojure.core/run!.

BUT is this warning even a problem anymore? Quick testing shows that with recent clojurescript and tooling this warning is no longer shown?

Solutions?

• Do nothing
• Rename

Notes:

• If renamed, we need first to deprecate the current name, add a new one, and keep the old one around with deprecation metadata and perhaps a runtime warning for at least one version
• The conflicting name is now only available in reagent.ratom. We could also expose this macro in reagent.core with a different name?
  • Users using the old one from ratom ns will need t