Add WriteV implementation for any Write objects

[jpopesculian]

Uses a write_vectored as a convenience function for implementing a WriteV adapter for any write object. It's a convenience function here, but I thought maybe this is food for thought in terms of changing write_tls to always use the write_vectored functionality if possible?

[coveralls]

Coverage remained the same at 96.49% when pulling 2a3296e on jpopesculian:writev_impl into 2164246 on ctz:master.

[codecov-io]

Codecov Report

Merging #296 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #296   +/-   ##
=======================================
  Coverage   96.48%   96.48%           
=======================================
  Files          50       50           
  Lines        8261     8261           
=======================================
  Hits         7971     7971           
  Misses        290      290

Impacted Files	Coverage Δ
rustls/src/vecbuf.rs	100% <ø> (ø)	⬆️

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[ctz]

Thanks!