AK: Bring back FixedArray<T>

Let's bring this class back, but without the confusing resize() API.
A FixedArray<T> is simply a fixed-size array of T.

The size is provided at run-time, unlike Array<T> where the size is
provided at compile-time.

Tests/AK/CMakeLists.txt

@@ -21,6 +21,7 @@ set(AK_TEST_SOURCES
     TestEndian.cpp
     TestEnumBits.cpp
     TestFind.cpp
+    TestFixedArray.cpp
     TestFormat.cpp
     TestGenericLexer.cpp
     TestHashFunctions.cpp