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/ccgcalls.nim

@@ -130,6 +130,7 @@ proc genPrefixCall(p: BProc, le, ri: PNode, d: var TLoc) =
   var length = sonsLen(ri)
   for i in countup(1, length - 1):
     assert(sonsLen(typ) == sonsLen(typ.n))
+    if ri.sons[i].typ.isCompileTimeOnly: continue
     if i < sonsLen(typ):
       assert(typ.n.sons[i].kind == nkSym)
       app(pl, genArg(p, ri.sons[i], typ.n.sons[i].sym))