Eth.Units.fromWei does not work with negative numbers

[cmditch]

> Maybe.map (EU.fromWei EU.Ether) (BigInt.fromString "20000000000000000000000")
Just "20000" : Maybe.Maybe String
> Maybe.map (EU.fromWei EU.Ether) (BigInt.fromString "-20000000000000000000000")
Just "000-20000" : Maybe.Maybe String
> 

[gary02]

so as with very big num:

> maxWei
 Pos (Magnitude [9639935,791312,4575840,564039,8466564,8532699,8687907,7098500,1954235,9237316,1579208,1])
> toString maxWei  
"115792089237316195423570985008687907853269984665640564039457584007913129639935"
> fromWei Gwei maxWei
"115792089237316195.129639935"
> fromWei Ether maxWei
"115792089.584007913129639935"

[gary02]

so as with very big num:

> maxWei
 Pos (Magnitude [9639935,791312,4575840,564039,8466564,8532699,8687907,7098500,1954235,9237316,1579208,1])
> toString maxWei  
"115792089237316195423570985008687907853269984665640564039457584007913129639935"
> fromWei Gwei maxWei
"115792089237316195.129639935"
> fromWei Ether maxWei
"115792089.584007913129639935"

elm-ethereum/src/Eth/Units.elm

Line 136 in e679669

-- There are under 10^27 wei in existance (so we safe for the next couple of millennia).

OK... maybe document should mention this detail