Switch to generics [internal]

[johnkerl]

Miller was ported to Go before generics existed, but they've been part of the Go language for quite a while now.

Main items:

• list.List -- awkward to genericize (namely Front, Back, Next) -- but it's better to simply use slices which are (and have always been) typed
• Genericize Miller's lib.OrderedMap

Why:

• Improved code legibility
• Moving from linked lists to sliced may positively impact performance due to simpler memory allocation
• Good development project