Fix quadratic behavior of repeated vectored writes

Some implementations of `Write::write_vectored` in the standard
library (`BufWriter`, `LineWriter`, `Stdout`, `Stderr`) check all
buffers to calculate the total length. This is O(n) over the number of
buffers.

It's common that only a limited number of buffers is written at a
time (e.g. 1024 for `writev(2)`). `write_vectored_all` will then call
`write_vectored` repeatedly, leading to a runtime of O(n²) over the
number of buffers.

The fix is to only calculate as much as needed if it's needed.

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