Type aliases for Vesta base and scalar fields

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Vesta base and scalar fields are equivalent to Pallas scalar and base fields corresponding. Therefore Pallas field types are used in Vesta curve implementations. Vesta field types are not added to Rust primitives.

But it may be convenient if we add type aliases for them to library, so users can use them explicitly:

type __zkllvm_field_vesta_base = __zkllvm_field_pallas_scalar;
type __zkllvm_field_vesta_scalar = __zkllvm_field_pallas_base;