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.

tests/run/ttypedesc1.nim

@@ -0,0 +1,35 @@
+import unittest
+
+type 
+  TFoo[T, U] = object
+    x: T
+    y: U
+
+proc foo(T: typedesc{float}, a: expr): string =
+  result = "float " & $(a.len > 5)
+
+proc foo(T: typedesc{TFoo}, a: int): string =
+  result = "TFoo "  & $(a)
+
+proc foo(T: typedesc{int or bool}): string =
+  var a: T
+  a = 10
+  result = "int or bool " & ($a)
+
+template foo(T: typedesc{seq}): expr = "seq"
+
+test "types can be used as proc params":
+  check foo(TFoo[int, float], 1000) == "TFoo 1000"
+  
+  var f = 10.0
+  check foo(float, "long string") == "float true"
+  check foo(type(f), [1, 2, 3]) == "float false"
+  
+  check foo(int) == "int or bool 10"
+
+  check foo(seq[int]) == "seq"
+  check foo(seq[TFoo[bool, string]]) == "seq"
+
+when false:
+  proc foo(T: typedesc{seq}, s: T) = nil
+