Simplify epochProcess creation

[dapplion]

Motivation

epochProcess creation is weird, it creates and empty object first to the mutate it. It would be simpler and easier to reason about to compute the values and return them as const directly.

Description

Return a new epochProcess object directly instead of mutate an empty object.

[codeclimate]

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[github-actions]

Performance Report

✔️ no performance regression detected

Full benchmark results

Benchmark suite	Current: 53d2db0	Previous: c09ea9a	Ratio
getCommitteeAssignments - req 1000 vs - 250000 vc	8.1562 ms/op	9.2084 ms/op	0.89
processBlock altair 250000 vs - 7PWei - maxed ops	487.11 ms/op	563.59 ms/op	0.86
processBlock altair 250000 vs - 7PWei - average	190.51 ms/op	220.05 ms/op	0.87
epoch altair - 250000 vs - 7PWei - processInactivityUpdates	2.5989 s/op	2.6754 s/op	0.97
epoch altair - 250000 vs - 7PWei - processRewardsAndPenalties	1.1092 s/op	1.1298 s/op	0.98
epoch altair - 250000 vs - 7PWei - processParticipationFlagUpdates	489.21 ms/op	578.71 ms/op	0.85
processBlock phase0 250000 vs - 7PWei - maxed ops	219.71 ms/op	240.26 ms/op	0.91
processBlock phase0 250000 vs - 7PWei - average	32.484 ms/op	30.951 ms/op	1.05
epoch phase0 - 250000 vs - 7PWei - beforeProcessEpoch	1.7458 s/op	2.0031 s/op	0.87
epoch phase0 - 250000 vs - 7PWei - processRewardsAndPenalties	876.92 ms/op	918.98 ms/op	0.95
epoch phase0 - 250000 vs - 7PWei - processEffectiveBalanceUpdates	134.04 ms/op	135.53 ms/op	0.99
getAttestationDeltas - 250000 vs - 7PWei	124.38 ms/op	124.54 ms/op	1.00
processSlots - 250000 vs - 7PWei - 32 empty slots	3.4435 s/op	3.6104 s/op	0.95
shuffle list - 16384 els	29.937 ms/op	30.941 ms/op	0.97
shuffle list - 250000 els	416.54 ms/op	298.32 ms/op	1.40
getPubkeys - persistent - req 1000 vs - 250000 vc	22.850 us/op	20.070 us/op	1.14
BLS verify - blst-native	2.3294 ms/op	2.3405 ms/op	1.00
BLS verifyMultipleSignatures 3 - blst-native	4.6112 ms/op	4.8100 ms/op	0.96
BLS verifyMultipleSignatures 8 - blst-native	9.9415 ms/op	10.355 ms/op	0.96
BLS verifyMultipleSignatures 32 - blst-native	35.995 ms/op	37.961 ms/op	0.95
BLS aggregatePubkeys 32 - blst-native	51.363 us/op	78.616 us/op	0.65
BLS aggregatePubkeys 128 - blst-native	190.87 us/op	210.14 us/op	0.91
getAttestationsForBlock	102.99 ms/op	89.045 ms/op	1.16
validate gossip signedAggregateAndProof - struct	5.3454 ms/op	5.7115 ms/op	0.94
validate gossip signedAggregateAndProof - treeBacked	5.3098 ms/op	5.6947 ms/op	0.93
validate gossip attestation - struct	2.4834 ms/op	2.6666 ms/op	0.93
validate gossip attestation - treeBacked	2.6817 ms/op	2.6806 ms/op	1.00

by benchmarkbot/action