encoding: add fast path of from/toutf8b() for ASCII strings See the previous patch for why. The added test seems not making much sense because ASCII strings should never contain "\xed" and be valid UTF-8. (with mercurial repo) $ export HGRCPATH=/dev/null HGPLAIN= $ hg log --time --config experimental.stabilization=all -Tjson > /dev/null (original) time: real 6.830 secs (user 6.740+0.000 sys 0.080+0.000) time: real 6.690 secs (user 6.650+0.000 sys 0.040+0.000) time: real 6.700 secs (user 6.640+0.000 sys 0.060+0.000) (fast jsonescape) time: real 5.630 secs (user 5.550+0.000 sys 0.070+0.000) time: real 5.700 secs (user 5.650+0.000 sys 0.050+0.000) time: real 5.690 secs (user 5.640+0.000 sys 0.050+0.000) (this patch) time: real 5.190 secs (user 5.120+0.000 sys 0.070+0.000) time: real 5.230 secs (user 5.170+0.000 sys 0.050+0.000) time: real 5.220 secs (user 5.150+0.000 sys 0.070+0.000)