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/tmemoization.nim

@@ -0,0 +1,17 @@
+discard """
+  msg:    "test 1\ntest 2"
+  output: "TEST 1\nTEST 2\nTEST 2"
+"""
+
+import strutils
+
+proc foo(s: expr{string}): string =
+  static: echo s
+
+  const R = s.toUpper
+  return R
+  
+echo foo("test 1")
+echo foo("test 2")
+echo foo("test " & $2)
+