buffer: remove _charsWritten

_charsWritten is an internal property that was constantly written to,
but never read from. So it has been removed.

Removed documentation reference as well.
test: Use 'close' event in simple/test-repl-timeout-throw
openssl: make sed tool configurable

Those poor sods on systems where /usr/bin/sed is a BSD sed can now do
`make -C deps/openssl/asm SED=gsed`.