SET statements: scope modifier for multiple assignments

[MohamedAbdeen21]

Follow up to both #1697 and #1694

Add scoping to a list of SET assignments (SET GLOBAL a = 1, LOCAL b = 2, ...).

Refactoring: remove duplicate code logic and simplify some logic.

Replace ContextModifier with Option and remove None variant.

[iffyio]

The changes look reasonable to me overall thanks @MohamedAbdeen21! left a comment re the representation of ContextModifier

[mvzink]

So I might be misunderstanding something, but I thought the reason this was a vec was to support some kind of SET var = a, b, c syntax, but I don't see that tested anywhere and don't remember where I saw it mentioned. If that syntax doesn't actually exist, could SingleAssignment be reworked to contain a SetAssignment or something?

[MohamedAbdeen21]

I believe you're referring to the test called parse_set_hivevar:

#[test]
fn parse_set_hivevar() {
    let set = "SET HIVEVAR:name = a, b, c_d";
    hive().verified_stmt(set);
}

The highlighted line is inside the block that parses a list of assignments and should fail for stmts like name = a, b since b by itself is not a valid assignment. Therefore, for this specific line, we always receive a single value and have to wrap it in a vec.

However, the code blocks (i.e. parsing rules) after this one produce a vec of values.

[MohamedAbdeen21]

For places where we produce a list of values, search for uses of parse_set_values.

[mvzink]

Got it, thank you!

[iffyio]

Thanks @MohamedAbdeen21! LGTM!
cc @alamb

[alamb]

Thank you all for keeping this train (of PRs) rolling. Very impressive I must say