only look up the format option from the hash once
shorten up IGNORE_OPTIONS

since we are now passing the format value around, we can remove it from
the options hash, which means we don't need to consult as many values
from IGNORE_OPTIONS
remove another value from IGNORE_OPTIONS
remove :as and :anchor from IGNORE_OPTIONS
add a test for missing "via" parameter
test with an empty via
always pull out a via variable and simplify logic
always make :via a list
pull up via extraction and remove it from options / IGNORE_OPTIONS
:on is removed from options before the Mapping is instantiated, so remove it from IGNORE_OPTIONS
disconnect the constraints method from the options and scope hashes
disconnect options and scope from the `blocks` method
disconnect normalize_defaults! from options[:constraints]
remove :constraints from IGNORE_OPTIONS
remove :defaults from the IGNORE_OPTIONS list
:only is never used in Mapping, so rm the key
remove IGNORE_OPTIONS
only look up scope[:module] once
no longer need the scope ivar
change defaults allocation to a one-liner