Added

• Tickrate type which contains a system and customFramerate.
• hertz, getTickrate procs to data module.
• items and mitems iterators for SongList

Changed

• Added skEnvelope to SequenceKind enum.
• Instrument type no longer contains the initEnvelope and envelope fields.
• Instrument now contains a sequence of type skEnvelope
• Song now has a tickrate field, for an optional tickrate override.
• File revision is now at 2.0, rev D.

Removed

• framerate proc in data module. Use getTickrate proc or Module.tickrate.