asyncjs: don't use JavaScript async

[zerbina]

Summary

• asyncjs no longer uses JavaScript's async
• result in .async procedures now works like it does everywhere else
• methods can no longer use .async

Details

• the goal is removing the usage of the deprecated .codegenDecl pragma
• routines marked with the .async pragma internally use a closure
  iterator that is then chained to a promise object when awaiting

---

To-Do

• make imported exceptions properly catchable within closure iterators (separate PR)
• re-add support for returning Futures from .async procedures
• write a proper commit message

Notes for Reviewers

• prior discussion about this can be found here
• an important prerequisite for the new JavaScript code generator

[saem]

Minor doc suggestion

[saem]

Suggested change

	## use a tool that backports the resulting JavaScript code, as babel.
	## use a tool that backports the resulting JavaScript code, such as babel.

[saem]

I know it's not strictly part of the change, but it was right there; not a requirement to update it.