transpile: Add Tagged wrapper to represent a potentially-unsafe expression

[Rua]

Unlike WithStmts, this doesn't contain any statements, but represents an expression alone. I've made it the return type of any functions that return an unsafe expression (notably, transmute_expr), and refactored all the call sites around the new return type. I've also added a Flattenable trait which makes things like and_then more generic; the genericised version is named flat_map and is implemented for WithStmts too.

My intent here is for WithStmts to eventually lose its is_unsafe field altogether, and for WithStmts<Tagged<T>> to be used instead in that role. I haven't done that here, as it will require more refactoring throughout the codebase.

The name Tagged is not particularly great, I'll admit. But I couldn't really come up with anything better at the moment. I considered naming it Safety, Unsafe or MaybeUnsafe, but that ties it too strongly to that particular function. It's possible that more fields could be added to it later, if anyone thinks of any. (Edit: just thought of another potential use, indicating whether the expression has side effects)