Remove legacy helpers

[mixonic]

Replaces #11858

Remove many legacy helper APIs, as well as Handlebars compile usage.

TODO:

• Deprecate block usage of unbound in 1.13
• Deprecate multiple arguments being passed to unbound in 1.13

[mmun]

ooooh, so many removals!

[rwjblue]

semantics for unbound are not the same with new helpers as with old ones. Causing test failures.

Yes, I noticed this as well. I'm perfectly fine with saying that it isn't intended to work, and therefore deprecating {{unbound}} + helper usage (which would make {{unbound}} for unbound props only).

[rwjblue]

Thank you @mixonic!