Move tracing "intercept" methods to _HookCaller?

[goodboy]

This relates to #217 as well as discussion in other issues.

The main refactoring would include:

• moving PluginManager._inner_hookexec lambda indirection onto the _HookCaller
• possibly (and subsequently) remove PluginManager._hookexec entirely
• re-work the tracing methods to support per hook tracing and if global tracing is desired have the manager loop through all underlying callers to insert trace functions.

[bluetech]

The first point is covered by #280.

For the second point I have a plan as well, will submit after the first PR is merged.

As for point 3, it should be possible, however the implementation plan of iterating over all _HookCallers is foiled by PluginManager.subset_hook_caller() which returns an untracked _HookCaller (and we probably don't want to track it, unless using weak references).