Fixed generic inheritances dispatch (nim-lang#5)

Using inherited generics does not dispatch properly and just takes the generic's base. This properly dispatches on the generic types so procs can overide based of inherited parameters. Making the following work:

```nim
type
  A[T] {.inheritable.} = object
  B = object of A[int]
  C = object of A[float]
proc doStuff(a: A[int]) = discard
proc doStuff(a: A[float) = discard
doStuff(B())
doStuff(C())
```

compiler/sigmatch.nim

@@ -1412,6 +1412,10 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
     let origF = f
     var f = if prev == nil: f else: prev
 
+    if a.len > 0:
+      let skippedA = a.skipTypesOrNil({tyObject}) # Skips if it's inheriting from a generic
+      if skippedA != nil:
+        a = skippedA
     let roota = a.skipGenericAlias
     let rootf = f.skipGenericAlias
 

tests/types/tinheritance_generic_dispatch.nim

@@ -0,0 +1,44 @@
+discard """
+  action: reject
+  cmd: '''nim check --hints:off $options $file'''
+  nimoutFull: true
+  nimout: '''
+tinheritance_generic_dispatch.nim(43, 5) Error: type mismatch: got <U>
+but expected one of:
+proc test(u: Union[string, RootObj])
+  first type mismatch at position: 1
+  required type for u: Union[system.string, system.RootObj]
+  but expression 'U()' is of type: U
+expression: test(U())
+tinheritance_generic_dispatch.nim(45, 6) Error: type mismatch: got <T>
+but expected one of:
+proc test2(u: Union[int, float])
+  first type mismatch at position: 1
+  required type for u: Union[system.int, system.float]
+  but expression 'T()' is of type: T
+expression: test2(T())
+tinheritance_generic_dispatch.nim(47, 6) Error: type mismatch: got <Union[system.string, system.RootObj]>
+but expected one of:
+proc test2(u: Union[int, float])
+  first type mismatch at position: 1
+  required type for u: Union[system.int, system.float]
+  but expression 'Union[string, RootObj]()' is of type: Union[system.string, system.RootObj]
+expression: test2(Union[string, RootObj]())
+'''
+"""
+
+type
+  Union[T, U] = object of RootObj
+
+  U = object of Union[int, float]
+  T = object of Union[string, RootObj]
+
+proc test(u: Union[string, RootObj]) = discard
+proc test2(u: Union[int, float]) = discard
+
+test(T())
+test(U())
+test(Union[string, RootObj]())
+test2(T())
+test2(U())
+test2(Union[string, RootObj]())