For functions, classes, and other items we often defer parsing their contents.
This struct captures the contents and the span at which they appeared.
It can then be used to parse the contents later.
We also use it to parse an expression like a[...],
as the ... may wind up being parsed as a generic term
or an expression.
Created when we parse x[..] expressions or paths to store the .. contents.
We canβt eagerly parse it because we donβt yet know whether to parse it
as types or expressions.