Compare buffers directly

[ryanio]

This PR focuses on this point from #775:

There's a huge number of unnecessary calls to toString(). Sometimes it is used instead of Buffer#equals. Sometimes it's called to initialize a BN from a Buffer, instead of just passing the Buffer.

These cases are handled in this PR:

• Block#validateTransactionsTrie
• Block#validateUnclesHash
• Account#isContract
• Account#setCode
• Account#isEmpty
• EEI#getExternalBalance
• PUSH's handler
• VM#runBlock
• VM#runCall
• VM#runTx
• DefaultStateManager#accountIsEmpty

These cases were not handled:

• Block#validateUncles: a Set is used to identify duplicates
• FakeTransaction#hash, Transaction#toCreationAddress: addressed in refactoring in Refactor tx lib #812

This PR also:

• Cleans up some found vars to let and const where appropriate
• Fixes outdated example decode-opcodes
• Updates ethash rawExample to new async syntax

[codecov]

Codecov Report

Merging #851 into master will increase coverage by 0.79%.
The diff coverage is 75.00%.

Flag	Coverage Δ
#account	92.85% <100.00%> (ø)
#block	?
#blockchain	81.33% <ø> (ø)
#common	94.50% <ø> (+0.17%)	⬆️
#ethash	83.33% <ø> (?)
#tx	94.02% <ø> (ø)
#vm	82.27% <71.42%> (-0.01%)	⬇️

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

[holgerd77]

Thanks Ryan, nice, love this kind of PRs! 🥳

Will merge.