Improve creation of target wrappers

[alexandrebouchard]

• ideally, would just pass in e.g. BridgeStan's StanModel directly. TODO: investigate if can reconstruct info passed n StanModel, and would have to modify how iid sampling handled with stan models (e.g. with an iid argument)
• alternative discussed was to have extra kargs passed directly
• or at least unify StanModel, etc into create_target (or potentially have first argument positional, with just target(...))

[alexandrebouchard]

...and at the same time, improve how examples/demos are handled. Stop exporting and use prefix convention for autocomplete Pigeons.demo_mvn(5)? example_target(:mvn, 5) ?