This change makes feeding output to commands which handle NUL terminated
"lines" (e.g., `fzf -read0` or `xargs -0`) extremely fast compared to
using an explicit Elvish loop that does `print $val"\x00"`. Similarly for
handling input from commands that produce NUL terminated "lines" (e.g.,
`find . -print0`) compared to an Elvish loop using `read-upto "\x00"`.
Resolves#1070
Related #1053
Also remove unused functions.
There were two (correct) implementations of deciding whether a string has
another string as a subsequence; keep the more performant one.
