replace unqualified core forms with qualified version and macro

[felix winkelmann]

These special forms should be replaced with macros that expand to qualified forms:

if
quote
syntax
let[rec]-syntax
define-[compiled-]syntax
foreign-[safe-]lambda[*]
foreign-primitive
define-foreign-variable
define-foreign-type
define-external-variable
define-constant
location

These should be then removed from canonicalization (with qualified versions remaining):

begin
let
letrec
lambda
set!

This should also take care of #18.