Maximize use of 'unowned' keyword.

Maximize use of 'unowned' keyword.

This is used when (and only when) variables:
1. would avoid a non-trivial copy (eg, a string or array but not an int or
object ref-count increase)
AND
2. will only be assigned unowned values (excessive use of 'unowned' here will be
caught by the compiler)

The net benefit is avoiding unecessary memory allocation/freeing.

Helps bgo#629083