Add `typed-index` feature

[toyboot4e]

It allows user to replace Index with Index<T> when they enable typed-index feature.

Changes:

• Added Index!(T) to lib.rs which conditionally refers to either Index or Index<T>
• Index or Index<T> was moved to index.rs (for no particular reason)
  • Index or Index<T> is crated with crate::index::create_index which conditionally returns Index or Index<T>

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I'm not sure if it's a good idea. Please do as you like :)

I just forked thunderdome and I don't need it actually.

[toyboot4e]

It was a very bad idea, since feature flags are additive!