[move-model-2] Switch maps to IndexMap

[tnowacki]

Description

• Module members are now ordered by their declared+compiled index
• This means docgen and other tools preserve the order declared by the user

Test plan

• 👀

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Release notes

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• Protocol:
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• CLI:
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[tzakian]

Overall looks good to me, although I wonder -- this will only keep relative declaration order but not full declaration order in docgen right? Or will it do full "declaration order" output? i.e.,

public struct B()
public enum A {
    C
}
public struct D()

In docgen will this give something like

public struct B() 
public struct D()
public enum A {
   C
}

or will this give

public struct B()
public enum A {
    C
}
public struct D()

[tnowacki]

For now

public struct B() 
public struct D()
public enum A {
   C
}

We could store the full index order in the info and use that if it is present, but that sounds like a different PR :)