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.h

@@ -32,7 +32,7 @@ public:
     virtual ErrorOr<void> write_until_depleted(ReadonlyBytes bytes) override;
 
     Bytes bytes();
-    ReadonlyBytes bytes() const;
+    ReadonlyBytes readonly_bytes() const;
     size_t offset() const;
     size_t remaining() const;