Implement (mostly) fmap2 for memory:

src/context/memory.rs

@@ -196,10 +196,16 @@ impl Grant {
     pub fn start_address(&self) -> VirtualAddress {
         self.start
     }
+    pub unsafe fn set_start_address(&mut self, start: VirtualAddress) {
+        self.start = start;
+    }
 
     pub fn size(&self) -> usize {
         self.size
     }
+    pub unsafe fn set_size(&mut self, size: usize) {
+        self.size = size;
+    }
 
     pub fn flags(&self) -> EntryFlags {
         self.flags