Remove or rationalize indexers

[jonsequitur]

Currently, the following classes have indexers:

• OptionResult (returns Option)
• CommandResult (returns Option)
• SymbolSet<T> (returns T, which can be Symbol or SymbolDefinition in various cases)

Their presence and behavior is not always clear. Should we remove them?

[jonsequitur]

Let's revisit this once we've had a larger discussion around improving the ParseResult API.

[jonsequitur]

Answer: Yes, we should remove them.