Remove @types/bn.js from block,blockchain,trie,tx,vm packages

[cgewecke]

Fixes a TS compilation failure on install relating to ethereumjs-util 7.0.9.

PR:

• removes @types/bn.js as a dependency everywhere
• imports bn.js via ethereumjs-util everywhere (for clarity about its source)

Idea is:

• bn.js is not a direct dependency in any package - it always comes from ethereumjs-util
• @types/bn.js is a direct dependency of ethereumjs-util
• monorepo should let ethereumjs-util manage the bn types

[codecov]

Codecov Report

Merging #1136 (78161f2) into master (183b04a) will increase coverage by 0.19%.
The diff coverage is 100.00%.

Flag	Coverage Δ
block	81.82% <ø> (+0.21%)	⬆️
blockchain	84.19% <ø> (+0.06%)	⬆️
client	86.84% <ø> (-0.20%)	⬇️
common	87.24% <ø> (?)
devp2p	84.40% <ø> (+0.36%)	⬆️
ethash	82.08% <ø> (ø)
tx	85.67% <100.00%> (+0.16%)	⬆️
vm	82.00% <ø> (ø)

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[ryanio]

lgtm, nice simplification :)

[holgerd77]

Ah, yeah, that's great. I wasn't aware that we have so many direct BN imports here. 😄 👍