Refactor kernel related code into a 'Kernel' subfolder off of 'src'

[rchiodo]

As discussed in our architecture review, this task is to move 'kernel' related code into a 'src/Kernels' folder (or whatever you decided to name it).

This would include stuff like:

• Kernel discovery
• Kernel execution (raw and native)
• Kernel dependency checks
• Kernel error checking
• IPywidget kernel pieces