Add an isa::lookup() function which serves as a target registry for creating
Box<TargetIsa> trait objects.
An isa::Builder makes it possible to confugure the trait object before it is
created.
- Move detail data structures into a settings::detail module to avoid polluting
the settings namespace.
- Rename generated data types to 'Flags' in anticipation of computed predicate
flags that can't be set. The Flags struct is immutable.
- Use a settings::Builder struct to manipulate settings, then pass it to
Flags::new().
This trait allows settings to be manipulated as strings, using descriptors and
constant hash-table lookups.
Amend gen_settings.py to generate the necessary constant tables.
