`num-bigint` does not compile, needs newer version

[muratbolu]

The num-bigint dependency seems buggy for version 0.3.2. Cloning and running cargo test does not work (on my machine) because rustc refuses to compile the old version. Running cargo update num-bigint@0.3.2 updates to v0.3.3 and seems to solve the issue. Running cargo update also works, but it seems to update many more packages.

[yigitsever]

Tests are not working, see #5

On my machine (rustc 1.72.0) gradecoin compiles without an issue. However, 1.78.0 does indeed not compile num-bigint.

Updating dependencies to an up-to-date version involves rewriting everything cryptography related, I'm aware of that