AccountSharedData::reserve: remove extra alloc + memcpy

Calling data_mut().reserve(additional) used to result in _two_ allocs
and memcpys: the first to unshare the underlying vector, and the second
upon calling reserve since Arc::make_mut clones so it uses capacity ==
len.

With this fix we manually "unshare" allocating with capacity = len +
additional, therefore saving on extra alloc and memcpy.

sdk/src/account.rs

@@ -543,7 +543,13 @@ impl AccountSharedData {
     }
 
     pub fn reserve(&mut self, additional: usize) {
-        self.data_mut().reserve(additional)
+        if let Some(data) = Arc::get_mut(&mut self.data) {
+            data.reserve(additional)
+        } else {
+            let mut data = Vec::with_capacity(self.data.len().saturating_add(additional));
+            data.extend_from_slice(&self.data);
+            self.data = Arc::new(data);
+        }
     }
 
     pub fn capacity(&self) -> usize {