[optimizer] don't merge A+B|B => A?+B for ChoiceRule

[Palatis]

we actually can't do this.

for A+B1|B2:

• when A matches, tries to match B1.
  • when B1 matches, A+B1 matches, and A+B1|B2 matches.
  • when B1 fails, A+B1 fails, fallback to B2
    • when B2 matches, A+B1|B2 matches
    • when B2 fails, A+B1|B2 fails.
• when A fails, try to match B2.
  • when B2 matches, A+B1|B2 matches
  • when B2 fails, A+B1|B2 fails.

however, for A?+B:

• when A matches, tries to match B
  • when B matches, A?+B matches
  • when B fails, fail. (no fallback path to match B only)
• when A fails, tries to match B
  • when B matches, A?+B matches
  • when B fails, fail.

the problem is for A?+B, when A matches, it wants to match a B, and success only when a valid B matches.
if A matches but B doesn't, A?+B fails.
this actually behaves different than A+B|B.

[Palatis]

some rules, for example CSharpGrammar.LambdaParameter is in this form.

public Rule LambdaParameter => Node(TypeExpr + Identifier | Identifier);

CSharpTests will fail if the code is re-written as

public Rule LambdaParameter => Node(TypeExpr.Optional() + Identifier);

for example:

public Rule LambdaParameter => Node(TypeExpr + Identifier | Identifier);
int a => "TypeExpr + Identifier" matches => passed
a => "TypeExpr + Identifier" fails, fallback to "Identifier" which matches => passed

public Rule LambdaParameter => Node(TypeExpr.Optional() + Identifier);
int a => "TypeExpr + Identifier" both match => passed
a => "TypeExpr" matches, but "Identifier" fails => failed