PARSER: add preliminary support for sequential destructuring forms with fn /
defn.
PARSER: add support for :as in sequential destructuring forms (used to retain
the original collection).
PARSER: optimize grammar rules for parsing destructuring forms.
PARSER: catch only IndexOutOfBoundsErrors when an argument to be destructured
has fewer elements than declared.
MISC: add functional tests for sequential destructuring forms.
PARSER: add support for map destructuring forms with fn / defn.
PARSER: add support for :keys, :strs in map destructuring forms.
MISC: add more functional tests for destructuring forms.
PARSER: add support for destructuring forms in let and loop bindings.