feat (Algebra/Vertex/HVertexOperator) : Composite heterogeneous vertex operators

[ScottCarnahan]

This PR defines the composite of two heterogeneous vertex operators as a heterogeneous vertex operator, using iterated Hahn series. Composite vertex operators appear in some definitions of vertex algebras.

Additional changes:

1. rm defeq abuse in coeff function.
2. Some necessary SMulZeroClass API for Hahn series is also added.

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