typedesc and expr params

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.

compiler/msgs.nim

@@ -663,7 +663,7 @@ proc InternalError*(errMsg: string) =
   writeContext(UnknownLineInfo())
   rawMessage(errInternal, errMsg)
 
-template AssertNotNull*(e: expr): expr =
+template AssertNotNil*(e: expr): expr =
   if(e == nil): InternalError($InstantiationInfo())
   e