version_49: Refactor stack writing code into a new StackWriter class (#1620)

This separates out the WasmBinaryWriter parts that do stack writing into a separate class, StackWriter. Previously the WasmBinaryWriter did both the general writing and the stack stuff, and the stack stuff has global state, which it manually cleaned up etc. - seems nicer to have it as a separate class, a class focused on just that one thing.

Should be no functional changes in this PR.

Also add a timeout to the wasm-reduce test, which happened to fail on one of the commits here. It was running slower on that commit for some reason, could have been random - I verified that general wasm writing speed is unaffected by this PR. (But I added the timeout to prevent future random timeouts.)