Reduce the number of dependencies

[vmx]

Triggered by #78 I found that there are more dependencies that can easily be removed and hence reduce the maintenance burden.

• bellperson is only needed for bellperson::groth16::aggregate::AggregateVersion. I suggest re-exporting it directly from filecoin-proofs, perhaps here: https://github.com/filecoin-project/rust-fil-proofs/blob/fbf3af067d9368fec2818f36fe2e2e7d5035d85b/filecoin-proofs/src/api/seal.rs. There we could add a re-export as pub roth16::aggregate::AggregateVersion. This would then bubble up automatically and could be used as use filecoin_proofs_v1::api::AggregateVersion.
• blstrs is only needed for blstrs::Scalar. It can also be re-exported from filecoin-proofs, just like the AggregateVersion and then be used as filecoin_proofs_v1::api::Fr.
• filecoin_hashers is only needed for filecoin_hashers::Hasher. Same thing as for the others, it would then be filecoin_proofs_v1::api::Hasher