WISH: implement collect and keep as new parse's keywords

[Oldes]

It's already implemented in Red language and quite often used, so it should be added to Rebol for a better compatibility.

In Red it works like:

>> parse "abc" [collect any [keep 1 skip]]
== [#"a" #"b" #"c"]

>> parse [1 "a"][collect any [integer! | keep string!]]
== ["a"]

>> parse "abc" [collect any [keep 1 2 skip]]
== ["ab" #"c"]

>> parse "abc" [collect any [collect any [keep 1 2 skip]]]
== [["ab" #"c"]]

>> parse "abc" [collect any [collect any [keep 2 skip] keep to end]]
== [["ab"] #"c"]

>>  parse "abc" [any [keep 1 skip]]
*** Script Error: PARSE - KEEP is used without a wrapping COLLECT

>> collect [parse "abc" [any [keep 1 2 skip]]] ;<--- requires parse's collect!
*** Script Error: PARSE - KEEP is used without a wrapping COLLECT