Be more strict about multiple indexes found in an archive

[raindev]

Fail fast rather than attempting to parse and overwriting already initialized symbol index. Came up in
#174 (comment).

[philipc]

This is basically fine, but you could consider using a single enum instead of multiple bools, and exposing that enum value could be useful too if users want to know which type of archive it is.

[raindev]

Using an enum to represent the state machine is definitely a clearer approach.

As for exposing the enum to the caller, I see the following options:

• change the return type of Archive::parse to (ArchiveType, Result<Archive<'a>>)
• include type: IndexType into Archive struct
• change Archive itself to be a enum

The second option seems preferable since the first incurs a breaking change. The third seems compelling conceptually since it'll allow to expose different data present in different archives in a type safe way while maintaining a common interface but it's quite a large change which includes many breaking changes to the interface of the library.

[philipc]

I prefer the second option. It makes sense to me for this to be an Archive property, rather than an additional return value, and I think changing Archive to an enum would lead to too much duplication.

[philipc]

Thanks!