Mark functions that return new instances

[yairm210]

E.g. .toMutableList()
These can then be autorecognized as local state, without explicit marking

[yairm210]

This can be put off until after Beta launch

[yairm210]

Expressions can be marked NewInstance if they are any of:

• Constructor
• Pure function call (e.g. listOf)
• @ReturnsNewInstance function call
• “newInstance” val

newInstance Vals are marked thus if the initializing expression is NewInstance

Functions marked @ReturnsNewInstance are checked that all ‘return’ values return NewInstance-type expressions

Open questions:

• Do we want to mark functions with annotations?
• Is there any way of getting this (essentially) "type information" in the IDE?