operator* and operator*=

[huyuguang]

the current code is:

    //Multiplication
    Bigint operator*(Bigint const &);
    Bigint &operator*=(Bigint const &);
    Bigint operator*(long long const &);
    Bigint &operator*=(int const &);

why not be "Bigint &operator*=(long long const &);" ?

[kasparsklavins]

Should be safe to change to long long.

[g19fanatic]

@kasparsklavins do you want pull requests with these types of changes or will you eventually add them yourself?