Move the 'new' out of the get_instance() methods and make them inline.
This has been replaced by a create_instance() method. This affects
all singleton classes (Hydrogen, AudioEngine?, Preferences, etc).
Related (and minor) things that are also in this commit:
* The get_instance() methods now have an assert() statement
in them. These either need to be removed later (after
the code is proven), or possibly add NDEBUG to our
release builds to remove them).
* Removed singleton stuff from H2Core::Sampler. It was not
being used.
* Removed singleton stuff from LashClient?. It was not being
used.
* Deleted the Sequencer class. It was not being used.
* Added a Mutex to MidiMap?. This may not have been necc., (was
added while chasing down the delete in MidiTable::saveMidiTable()),
But, it seems appropriate that it have one.
