chimw: the chi-shaped helpers of the custom services

GetHead registers a read route under GET and HEAD both, which cover, bench
and tokens had each spelled out — five copies of three lines, under three
names. RenderRefusals points chi's NotFound and MethodNotAllowed at the
service's own error page, which spec and dolt install neither of, so a
mistyped URL there is the one refusal on the instance that answers in
plain text. RequestLogger is a chi LogFormatter emitting slog records, in
place of chi's colourised stdlib line on stdout — the highest-volume line
a service writes and the only one that was neither structured nor on
stderr.

It is a second middleware package and not a wing of the first because
sr-ht-ecore/middleware needs nothing but net/http and that rule is worth
keeping; the boundary is whether the helper has to know what a route is.

A request line is Info, a 5xx is Error, and 499 stays Info by being below
500, which is what that code was chosen for. Skipping probe noise is the
caller's call, through a predicate. The logger goes outside RecoverPanics
so the line reports the error page that was rendered rather than the
nothing an unwinding stack has written.