* Configuration system. Kill DFL_T
* add circ::cfg::CircCfg that holds cfg info
* it's constructible from circ_opt::CircOpt
* implements clap::Args, so you can set it from your compiler's
CLI/envvars
* defined in external crate to keep clap out of our main build
* organized by circ module, but not feature gated
* no point: the build wouldn't meaningfully change
* includes a way to set the default field
* added circ::cfg::set and circ::cfg::cfg
* also circ::cfg::set_default and circ::cfg::set_cfg
* access a sync::once_cell, static configuration
* killed DFL_T
* workflows
* unit-tested component probably need to not read circ::cfg::cfg.
* compilers need to call circ::cfg::set or circ::cfg::set_default.
* rm dead features
