encoding: add function to test if a str consists of ASCII characters
Most strings are ASCII. Let's optimize for it.
Using uint64_t is slightly faster than uint32_t on 64bit system, but there
isn't huge difference.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2