std: Implement `LineWriter::write_vectored`

This commit implements the `write_vectored` method of the `LineWriter`
type. First discovered in bytecodealliance/wasmtime#629 the
`write_vectored` method of `Stdout` bottoms out here but only ends up
writing the first buffer due to the default implementation of
`write_vectored`.

Like `BufWriter`, however, `LineWriter` can have a non-default
implementation of `write_vectored` which tries to preserve the
vectored-ness as much as possible. Namely we can have a vectored write
for everything before the newline and everything after the newline if
all the stars align well.

Also like `BufWriter`, though, special care is taken to ensure that
whenever bytes are written we're sure to signal success since that
represents a "commit" of writing bytes.

0 files changed, 0 insertions, 0 deletions