Add a function for retrieving all saved metadata of the current binary view in a dictionary

[kr1tzy]

Right now the only way to retrieve all of the saved metadata for the current binary view is to bv.query_metadata(key) each of the saved keys. This requires manually updating a static list for each new key and consolidating dictionaries. Having something like bv.get_all_metadata() return a dictionary of every key-value pair would make this task much smoother.