Document differences from actual Ivy

[dijkstracula]

Should compile a list of explicit differences and put them somewhere.

• No parameterized objects using instance; only process is supported.
• Extract isn't a thing; all objects are implicitly extracted.

[dijkstracula]

• A number is automatically inferred to be a numeric sort synonymous with unbounded_sequence, and we have a subtyping relationship to unify variables typed to an uninterpreted sort with numeric expressions. In Ivy, by contrast, numbers' sorts are inferred entirely by context.

[dijkstracula]

No structs.