Adding `showCollections()` and `showDatabases()` methods.

[ralmond]

In the mongo shell, I frequently find myself using the show databases and show collections commands. (It is really useful for debugging situations where I have misspelled or misremembered a database or collection name).

You demonstrate how to do this in the manual using the $run() command, but they are probably used often enough to warrant their own methods.

[jeroen]

I suppose this should wrap:

• https://mongoc.org/libmongoc/current/mongoc_client_get_database_names_with_opts.html
• https://mongoc.org/libmongoc/current/mongoc_database_find_collections_with_opts.html

[ralmond]

At a quick look, that looks good. (I haven't looked at the C API in detail).