Work-in-progress PR. Submit to upstream once it's more complete.

Changes:

• Create the ops.CharmMeta object using the dictionaries that we already have in the _CharmSpec object rather than loading and deserialising from disk.
• Cache the metadata, config, and actions dictionaries that are loaded from charmcraft.yaml (or the separate YAML files) in the original charm directory.

It would be nice if we could avoid deserialising and writing the YAML files to the temporary charm directory. However, charms might be using those directly (I think we can say that they shouldn't for metadata.yaml, and probably actions.yaml - however CharmMeta does not currently load config.yaml, so it's legitimate to do that to get the defaults, for example). We could certainly cache the serialised form - we could perhaps symlink rather than write, but since the original form is possibly charmcraft.yaml, we'd need to write to one place, so that might not be a win. Biggest question is whether the charm tests actually need these files.

At the moment, timing is not necessarily a win:

Once done, should resolve canonical#1498

There are some drive-by trailing whitespace fixes that come from running the pre-commit linting in the testing folder. Could move these out to a separate PR.

Similarly, shebangs are removed from files that aren't meant to be directly executed. Could move those out too.