Simplify enum syntax to a single line

[richardpawson]

from:

enum Algorithm
  aStar, dijkstra, heuristic
end enum

to:

enum Algorithm set to aStar, dijkstra, heuristic

Rationale:

• Most enums have a short list of values, so will fit on one line
• New syntax looks more like constant declaration. Difference is still that it is a Type, so needs to be capitalised.
• enum...end enum suggests that it can hold multiple frames when it can't (it is always just one frame).