Null Coalescence operator

[Knetic]

A coalescence operator ?? should be added. It ought to share precedence with ternaries. It should return the left-hand value if it is not nil, otherwise the right-hand value (regardless of nil). This will mirror the C# / ECMA standard of null coalescence, without making users resort to ugly ternary conditionals for the same effect.

[Knetic]

Done as of 2923a49