Add `Pickable` implementation for `bool` and the supporting `Matcher`
trait
with utility functions for argument matching.  Expose `pickable_needed`
and
`matcher_needed` modules for downstream trait implementors.

Rename the `core` feature to `mingling_support` in `mingling_picker` and
propagate it through `mingling_picker_macros`.  Add the `is_flag` field
to `PickerTag` to indicate participation in parsing after a `--`
separator.  Remove empty `seek.rs` file.

Define a `core` feature in mingling_picker that gates a new `corebind`
module and re-exports its contents. The module is propagated to the
upstream `mingling` crate under the same feature flag.