fixes #8519; implements T.distinctBase to reverse T = distinct A (#8531)

tests/stdlib/tsugar.nim

@@ -0,0 +1,29 @@
+discard """
+  file: "tsugar.nim"
+  output: ""
+"""
+import sugar
+import macros
+
+block distinctBase:
+  block:
+    type
+      Foo[T] = distinct seq[T]
+    var a: Foo[int]
+    doAssert a.type.distinctBase is seq[int]
+
+  block:
+    # simplified from https://github.com/nim-lang/Nim/pull/8531#issuecomment-410436458
+    macro uintImpl(bits: static[int]): untyped =
+      if bits >= 128:
+        let inner = getAST(uintImpl(bits div 2))
+        result = newTree(nnkBracketExpr, ident("UintImpl"), inner)
+      else:
+        result = ident("uint64")
+
+    type
+      BaseUint = UintImpl or SomeUnsignedInt
+      UintImpl[Baseuint] = object
+      Uint[bits: static[int]] = distinct uintImpl(bits)
+
+    doAssert Uint[128].distinctBase is UintImpl[uint64]