* Sync with upstream. I/O callbacks are now consolidated into a single read callback and a single write callback, with an access type specifier.
* Initial working implementation of YM278B. Most features implemented, except vibrato.
* Implement vibrato and status register flags. Fix envelope rate computation.
* Rename ymfm_interface::external_type to access_class and clean up the fallout.
* Formally replace the old YMF278B engine with the one from ymfm
* Rotated YMF278B outputs into a more logical order.
* Re-evaluted envelope calculations and 2x works better than the weird 15/8 I came up with before. Also changed the way FM resampling is computed to be more precise (and simpler). Turned off extraneous debugging.
* Start of/reset to a null state with no loaded waveforms.
* Fix YM2608 I/O ports.
ymfm: refactor the code into a separate 3rdparty library
* Moved ymfm core implementation to 3rdparty/ymfm
* Split out each family (OPM/OPN/OPL/etc) into its own source file
* Added preliminary OPQ and OPZ support, still WIP
* Put all 3rdparty code into its own namespace ymfm
* Fixed various bugs reported in #8042
* Created interface class for communication between the 3rdparty engine and the emulator
* Standardized MAME implementation of all Yamaha devices based on a template class
* Created standard base class ym_generic that can be used when multiple YM chips are swapped in
* Changed YM2203/2608/2610 to embed a YM2149 as a subdevice instead of deriving from ay8910_device
* Also provided compile-time option to use a simplified built-in SSG rather than using MAME's at all (currently off)
* Consolidated MAME header files from one-per-chip (ym2151.h, ym2203.h, etc) to one-per-family (ymopm.h, ymopn.h, etc)
