We can avoid a slow regexp capture and instead and rely on
rindex + substr to extract the section from the config file.
Then we use the defined-or-assignment (//=) operator combined
with the documented return value of `push' to ensure @section_order
is unique without repeating a hash lookup.
Finally, we avoid short-lived variables inside the loop and
declare them subroutine-wide to knock a teeny bit of allocation
time.
Combined, these optimizations bring the ~1.22s
PublicInbox::Config->new time down to ~0.98s with 50K inboxes.