Having installed bignum (from https://github.com/FedeOmoto/bignum) I'm getting this:
C:UsersPete.nimblepkgsbignum-1.0.0bignumprivateint.nim(1, 8) Error: cannot open file: unsigned
"unsigned" is an old module which has been deprecated and no longer exists.
I’m surprised by this error as I have "bignum" installed on my machine and didn’t encounter any problem.
The last version of "bignum" is 1.0.4, yours seems to be 1.0.0, so the problem may be just to install the most recent version. But, strangely, on github the first line in "int.nim" is "import unsigned" which is indeed wrong. This error doesn’t exist in the version I installed using "nimble", so I think you should install it this way by typing nimble install bignum.
Anyway, there is something not clear in the github version.
Note that it may be enough to delete the faulty import to solve the problem.