GtkSerializer should use transcoding stream

[Mrnikbobjeff]

Description

The GtkSerializer does implement transcoding inefficiently. Instead of reading the entire value into memory, one could simply peek the stream for the fast path instead of reading it in one go. Furthermore, instead of materializing the result before calling the deserializer one could use the Encoding.CreateTranscodingStream method to have a stream which automatically encodes the input to UTF8.
Recreation of issue with correct GH account

Public API Changes

None

Intended Use-Case

Allocation reduction