Make _UInt128 a little more like UInt128

Author: tbkka

stdlib/public/core/LegacyInt128.swift.gyb

@@ -21,9 +21,11 @@ internal struct _${U}Int128 {
 
   /// The low part of the value.
   internal var low: Low
+  internal var _low: Low { low }
 
   /// The high part of the value.
   internal var high: High
+  internal var _high: High { high }
 
   /// Creates a new instance from the given tuple of high and low parts.
   ///
@@ -39,6 +41,11 @@ internal struct _${U}Int128 {
     self.high = high
   }
 
+  internal init(_low: Low, _high: High) {
+    self.low = _low
+    self.high = _high
+  }
+
   internal init() {
     self.init(high: 0, low: 0)
   }