Support multiple BLS implementations (#1335)

## Issue Addressed

NA

## Proposed Changes

- Refactor the `bls` crate to support multiple BLS "backends" (e.g., milagro, blst, etc).
- Removes some duplicate, unused code in `common/rest_types/src/validator.rs`.
- Removes the old "upgrade legacy keypairs" functionality (these were unencrypted keys that haven't been supported for a few testnets, no one should be using them anymore).

## Additional Info

Most of the files changed are just inconsequential changes to function names.

## TODO

- [x] Optimization levels
- [x] Infinity point: https://github.com/supranational/blst/issues/11
- [x] Ensure milagro *and* blst are tested via CI
- [x] What to do with unsafe code?
- [x] Test infinity point in signature sets

consensus/state_processing/src/per_block_processing.rs

@@ -3,7 +3,6 @@ use errors::{BlockOperationError, BlockProcessingError, HeaderInvalid, IntoWithI
 use rayon::prelude::*;
 use safe_arith::{ArithError, SafeArith};
 use signature_sets::{block_proposal_signature_set, get_pubkey_from_state, randao_signature_set};
-use std::convert::TryInto;
 use tree_hash::TreeHash;
 use types::*;
 
@@ -215,7 +214,7 @@ pub fn verify_block_signature<T: EthSpec>(
             block_root,
             spec
         )?
-        .is_valid(),
+        .verify(),
         HeaderInvalid::ProposalSignatureInvalid
     );
 
@@ -235,8 +234,7 @@ pub fn process_randao<T: EthSpec>(
     if verify_signatures.is_true() {
         // Verify RANDAO reveal signature.
         block_verify!(
-            randao_signature_set(state, |i| get_pubkey_from_state(state, i), block, spec)?
-                .is_valid(),
+            randao_signature_set(state, |i| get_pubkey_from_state(state, i), block, spec)?.verify(),
             BlockProcessingError::RandaoSignatureInvalid
         );
     }
@@ -452,7 +450,7 @@ pub fn process_deposit<T: EthSpec>(
     // depositing validator already exists in the registry.
     state.update_pubkey_cache()?;
 
-    let pubkey: PublicKey = match (&deposit.data.pubkey).try_into() {
+    let pubkey: PublicKey = match deposit.data.pubkey.decompress() {
         Err(_) => return Ok(()), //bad public key => return early
         Ok(k) => k,
     };