The terminal.Stream next/nextSlice functions can now no longer fail.
All prior failure modes were fully isolated in the handler `vt`
callbacks. As such, vt callbacks are now required to not return an error
and handle their own errors somehow.
Allowing streams to be fallible before was an incorrect design. It
caused problematic scenarios like in `nextSlice` early terminating
processing due to handler errors. This should not be possible.
There is no safe way to bubble up vt errors through the stream because
if nextSlice is called and multiple errors are returned, we can't
coalesce them. We could modify that to return a partial result but its
just more work for stream that is unnecessary. The handler can do all of
this.
This work was discovered due to cleanups to prepare for more C APIs.
Less errors make C APIs easier to implement! And, it helps clean up our
Zig, too.
This works around: https://github.com/ziglang/zig/issues/19148
This lets our `test-valgrind` command catch some issues. We'll have to
follow this pattern in more places but I want to do it incrementally so
things keep passing.
I **do not** want to blindly follow this pattern everywhere. I want to
start by focusing in only on the structs that set `undefined` as default
fields that we're also about to test in isolation with Valgrind. Its
just too much noise otherwise and not a general style I'm sure of; it's
worth it for Valgrind though.
