AK: Rename the const overload of FixedMemoryStream::bytes()

Due to overload resolutions rules, this simple code provokes a crash:

ReadonlyBytes readonly_bytes{};
FixedMemoryStream stream{readonly_bytes};
ReadonlyBytes give_them_back{stream.bytes()};
    // -> Panics on VERIFY(m_writing_enabled);
    // but this is fine:
auto bytes = static_cast<FixedMemoryStream const&>(*stream).bytes()

If we need to be explicit about it, let's rename the overload instead of
adding that `static_cast`.

AK/MemoryStream.cpp

@@ -114,7 +114,7 @@ Bytes FixedMemoryStream::bytes()
     VERIFY(m_writing_enabled);
     return m_bytes;
 }
-ReadonlyBytes FixedMemoryStream::bytes() const
+ReadonlyBytes FixedMemoryStream::readonly_bytes() const
 {
     return m_bytes;
 }