tx/vm/util: throw when provided negative BN

[gabrocheleau]

This PR resolves #1513 with the following changes:

• tx package: Throw when constructing a Transaction with a negative BigNumber value (in toBuffer)
• vm package: Throw when passed a negative call value (throws in the Message constructor)

As mentioned in a comment of the above issue, the toBuffer method also has the side effect of converting negative BN to positive BN. I decided to throw an error in the case of negative BN in that method, since it seemed to me like that behavior isn't intended.

I was not sure about disabling negative BNs at the module level, so omitted that for now.

[codecov]

Codecov Report

Merging #1606 (420328c) into master (d390b9b) will increase coverage by 0.32%.
The diff coverage is 100.00%.

Flag	Coverage Δ
block	84.96% <ø> (ø)
blockchain	85.08% <ø> (?)
client	71.27% <ø> (ø)
common	100.00% <ø> (ø)
devp2p	82.82% <ø> (-0.21%)	⬇️
ethash	∅ <ø> (?)
trie	86.09% <ø> (ø)
tx	88.62% <ø> (ø)
util	91.84% <100.00%> (+0.03%)	⬆️
vm	79.89% <100.00%> (?)

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

[ryanio]

i'm seeing uses of both "can not" and "cannot" in this PR, let's just use "cannot"

Both cannot and can not are perfectly fine, but cannot is far more common and is therefore recommended, especially in any kind of formal writing.
https://www.merriam-webster.com/words-at-play/cannot-vs-can-not-is-there-a-difference

[ryanio]

lgtm! will wait if you want to try to add a negative value tx to runTx (using value=0 and tx.value.isubn(1) as jochem suggested)

[ryanio]

looks great! can merge after ci passes