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/cgen.nim

@@ -606,6 +606,7 @@ proc genProcAux(m: BModule, prc: PSym) =
         res.loc.s = OnUnknown
   for i in countup(1, sonsLen(prc.typ.n) - 1): 
     var param = prc.typ.n.sons[i].sym
+    if param.typ.isCompileTimeOnly: continue
     assignParam(p, param)
   closureSetup(p, prc)
   genStmts(p, prc.getBody) # modifies p.locals, p.init, etc.