* Add APIs to lookup values in `Linker`
This commit adds three new methods to `Linker` in order to inspect it
after values have been inserted:
* `Linker::iter` - iterates over all defined values
* `Linker::get` - lookup a value by its `ImportType`
* `Linker::get_by_name` - lookup values based on their name
Closes#1454
* More apis!