We have zero-cost conversion methods for converting e.g. From<Expression> for Argument. It's zero cost because Argument inherits Expression's variants, so a valid Expression is also a valid Argument, and the conversion is just an in-place transmute.

Implement the same for converting Vecs. e.g. Vec<'a, Expression<'a>> should be able to be converted to Vec<'a, Argument<'a>> as a zero cost transmute - without memory copy or allocation.

Ditto From<Box<'a, Expression<'a>>> for Box<'a, Argument<'a>>. Edit: No point. We never box enums.

The code touched in oxc-project/oxc#7854 is an example of where we could use this for a more efficient operation.