Mark gset methods as simpl never to stop `cbn` from unfolding them

This clarifies what's meant to happen anyway, and is apparently needed for cbn. (I haven't yet tried doing this at the mapset level).

Example of the problem: without this change, cbn mangles the goal in the following:

  Goal {[1; 2; 3]} =@{gset nat} ∅.
  Fail progress simpl.
  progress cbn. Show.

I'd want progress cbn to fail, instead the above gives:

The command has indeed failed with message:
Failed to progress.
1 subgoal
  
  ============================
  {| mapset_car := {[1 := ()]} ∪ {[2 := ()]} ∪ {[3 := ()]} |} = ∅

tests/gmap.ref

+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  {[1; 2; 3]} = ∅
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  elements {[1; 2; 3]} = []
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  {[1; 2; 3]} ∖ {[1]} ∪ {[4]} ∩ {[10]} = ∅ ∖ {[2]}
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  1 ∈ dom (gset nat) (<[1:=2]> ∅)
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  bool_decide (∅ = {[1; 2; 3]}) = false
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  bool_decide (∅ ≡ {[1; 2; 3]}) = false
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  bool_decide (1 ∈ {[1; 2; 3]}) = true
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  bool_decide (∅ ## {[1; 2; 3]}) = true
+The command has indeed failed with message:
+Failed to progress.
+The command has indeed failed with message:
+Failed to progress.
+1 goal
+  
+  ============================
+  bool_decide (∅ ⊆ {[1; 2; 3]}) = true