[Mammoth] Add the ability to duplicate an owned memory region.

lib/mammoth/util/memory_region.cpp

@@ -71,4 +71,8 @@ z_cap_t OwnedMemoryRegion::DuplicateCap() {
   return cap;
 }
 
+OwnedMemoryRegion OwnedMemoryRegion::Duplicate() {
+  return OwnedMemoryRegion::FromCapability(DuplicateCap());
+}
+
 }  // namespace mmth

lib/mammoth/util/memory_region.h

@@ -30,6 +30,7 @@ class OwnedMemoryRegion {
 
   z_cap_t cap() const { return vmmo_cap_; }
   z_cap_t DuplicateCap();
+  OwnedMemoryRegion Duplicate();
 
   bool empty() const { return vmmo_cap_ == 0; }
   explicit operator bool() const { return vmmo_cap_ != 0; }