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.