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/ast.nim

@@ -711,6 +711,15 @@ proc `[]`*(n: PNode, i: int): PNode {.inline.} =
 var emptyNode* = newNode(nkEmpty)
 # There is a single empty node that is shared! Do not overwrite it!
 
+proc linkTo*(t: PType, s: PSym): PType {.discardable.} =
+  t.sym = s
+  s.typ = t
+  result = t
+
+proc linkTo*(s: PSym, t: PType): PSym {.discardable.} =
+  t.sym = s
+  s.typ = t
+  result = s
 
 const                         # for all kind of hash tables:
   GrowthFactor* = 2           # must be power of 2, > 0