libluna: Add the clear_data() method to Vector and use it to optimize Base64::decode

This method clears the Vector's data without deallocating the
backing buffer, so that it can be reused without reallocation.

libluna/include/luna/Vector.h

@@ -201,6 +201,13 @@ template <typename T> class Vector
         m_data = nullptr;
     }
 
+    void clear_data()
+    {
+        for (usize i = 0; i < m_size; i++) { m_data[i].~T(); }
+
+        m_size = 0;
+    }
+
     Result<Vector<T>> shallow_copy()
     {
         Vector<T> other;

libluna/src/Base64.cpp

@@ -118,6 +118,7 @@ namespace Base64
         }
 
         Vector<char> chars_read;
+        TRY(chars_read.try_reserve(4));
 
         for (const auto& c : data)
         {
@@ -134,7 +135,7 @@ namespace Base64
             if (chars_read.size() == 4)
             {
                 TRY(decode_base64_buffer(chars_read, buf));
-                chars_read.clear();
+                chars_read.clear_data();
             }
         }