GlobRef: Remove reference-based comparison and postfix operators

[devreal]

Changes to GlobRef:

1. Remove reference-based comparison operator to perform value-based comparison and allow a test like arr[0] == arr[1] to succeed. Add test. See also Fix GlobAsyncRef #447
2. Remove postfix increment/decrement operators because their semantic cannot be correctly implemented, i.e., (arr[0]++)++ will not perform two increments and return the result of the first increment. It's better to error-out than to yield incorrect results. After thinking about this again I realized that it's perfectly reasonable to have these operators on global references as they return a value and not a reference and (arr[0]++)++ is illegal anyway. Re-added the operators.

[devreal]

Fix for the failing test case is contained in #447

[fuchsto]

The increment-operator issue is a really good catch. The same situation with operator+=, and any operator@= for that matter. For asyncs, these should return a future on the operation. I will meditate on that.

[devreal]

The same situation with operator+=, and any operator@= for that matter.

Not sure I can follow: operator+= should return the result of the addition-assignment so returning a reference should work as expected. The catch really only is the postfix increment since that is supposed to return the previous state, which cannot be done by returning a reference.