get rid of confstruct

[jrochkind]

confstruct has not lived up to it's promise. it regularly causes me very difficult to debug problems.

i'd like to remove it, but that will be disruptive, as it will be hard to construct configuration api that is identical (even for common use cases) without it, and I don't really want a drastic change to configuration api.

either way, probably waits for the next major version.