feature request: implement quoted extensions

[bobzhang]

in ocaml trunk

{%raw||}

is translated into [%raw{||}]
This is a nice addition, since it is pure syntax, we can implement it today

[bobzhang]

relevant issue: ocaml/ocaml#8820

[davesnx]

Hey @bobzhang, happy to take a look at this. Would you mind giving me a starting point?

Thanks