Update to Zig 0.15

[tranzystorekk]

Ensured that zig build test passes on Zig 0.15.1, also some misc mentions:

• added a flush point at the end of serialize, since writers may be buffered now and I assumed we don't want to leave the serialized data waiting in buffer for a manual flush from the user
• the serialize tests now use a discarding writer, since I believe we only want to check for unexpected errors in the serialization process, and actually writing the data anywhere is unnecessary