sfeed_plain: optimize utf8-decoding and column position calculation

Optimize for the common-case: assuming ASCII.
The input is assumed to be valid UTF-8 input (output of sfeed).

This saves 2 function calls for determining the width of a single ASCII
character, which of course is 1.

Ranges:
< 32 are control-characters and are skipped.
< 127 is typical ASCII and is 1 column wide.
>= 127 is the normal path (control-character and multi-byte UTF-8).

Tested on OpenBSD and Linux with various compilers (clang, gcc, pcc and tcc).
On OpenBSD and Linux glibc much improvement. On Linux musl (almost) no change.

In a common-case upto 40% performance improvement.
In the worst-case negligible performance degration (<1%).

0 files changed, 0 insertions, 0 deletions