Add 'apply' statement to Plan language

Finalize the apply keyword and get it merged into master of puppet.

Finalize grammar: the following are all allowed signatures

apply(TargetSpec $targets) { ... }

apply(TargetSpec $targets, Hash[String, Data] $options) { ... }

apply(TargetSpec $targets, String $description) { ... }

apply(TargetSpec $targets, String $description, Hash[String, Data] $options) { ... }

Note that type checking doesn't happen until evaluation time. Right now it's easiest to do that in the apply_executor callback. Allow multiple arguments.

Write unit tests.

WIP lives in the apply-grammar branch of Puppet.