types are now valid proc/template/macro params and you can overload over them:
proc foo(T: typedesc) # accept any type
proc foo(T: typedesc{int}) # overload specifically for int
proc foo(T: typedesc{int or float or Callable}) # overload for any type matching the constraints
expr{type} is a param expecting compile time value of the designated type (or type class).
when typedesc or expr params are used with a proc, the proc will be instantiated once
for each unique type/value used as parameter.
