block: make withdrawals trie check consistent with txtrie

[jochem-brouwer]

When working on #3336 I noticed some inconsistent things unrelated to EIP 7002.

This PR fixes that. (Have currently set it up to not run CI yet since I might want to add other things I find)

Currently:

• Adds withdrawalsTrie cache (we do not want to re-calculate the withdrawals trie each time)
• Speeds up withdrawals trie calculation if there are no withdrawals
• Speeds up uncle hash calculation if there are no uncles (now each time there are no uncles we return return equalsBytes(this.keccakFunction(RLP.encode([])), this.header.uncleHash), this is unnecessary especially since we currently have no uncles)

[acolytec3]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.92%. Comparing base (ad6c603) to head (65544f2).
Report is 1 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
block	82.24% <100.00%> (?)
blockchain	90.92% <ø> (ø)
client	84.92% <ø> (?)
common	94.02% <ø> (ø)
devp2p	81.36% <ø> (ø)
evm	73.30% <ø> (?)
genesis	99.98% <ø> (ø)
statemanager	74.71% <ø> (ø)
trie	59.60% <ø> (-0.08%)	⬇️
tx	85.70% <ø> (ø)
util	81.32% <ø> (ø)
vm	61.56% <ø> (ø)
wallet	87.25% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[acolytec3]

LGTM