refactor(Data/Seq): reorganize Seq.lean (leanprover-community#20071)

The `Seq` API in the `Seq.lean` file was somewhat disorganized. This PR reorganizes the file to make it more structured.  

It arranges the API in the following way:
1. Definitions and the `get?` function.  
2. Constructors: `nil` and `cons`.  
3. Destructors: `head`, `tail`, and `destruct`.  
4. Recursion and corecursion principles.  
5. Bisimulation.  
6. `Terminates` API.  
7. `Membership` for sequences.  
8. Conversion to and from other types (`List`, `Stream`, `MLList`) and `take` operation as it is required for conversion to `List`.  
9. Definitions of other operations on sequences (`append`, `map`, `join`, etc.).  
10. Lemmas about conversions and operations, organized into sections by the primary operation of each lemma.

Author: vasnesterov