fixes #797; generic procs can be used in places expecting matching concrete proc types

tests/generics/tinferredgenericprocs.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''123
+1
+2
+3'''
+"""
+
+# https://github.com/Araq/Nimrod/issues/797
+proc foo[T](s:T):string = $s
+
+type IntStringProc = proc(x: int): string 
+
+var f1 = IntStringProc(foo)
+var f2: proc(x: int): string = foo
+var f3: IntStringProc = foo
+
+echo f1(1), f2(2), f3(3)
+
+for x in map([1,2,3], foo): echo x
+