Consider making a `CowModuleBuilder` abstraction

> Maybe a nice abstraction here, and to make the `CowSection` constructors nicer per other comment while keeping this code clean too, would be a `CowModuleBuilder`: wrap the `Vec`, and have methods for `add_owned(...)` and `add_borrowed(...)`, and then `into_module()` or somesuch?

_Originally posted by @cfallin in https://github.com/fitzgen/winliner/pull/2#discussion_r1333598093_