Implement mincostflow

[kenkoooo]

Port mincostflow from the AtCoder's library, resolved #21.

Questions

• It handles Cap and Cost as the same type. Is it better to separate types?
  • => Let's keep it for now.
• It uses Integer trait. Does it make sense?
  • => After we finished #19, let's replace the old one with the new one.
• We can change some variable names. Let me hear your opinion.

[MiSawa]

For Cap and Cost -- in theory we can use a different type for Cap, Cost and Result type where Result : From<Cap> + From<Cost>.... which can be useful for problems like this, though probably overkill for almost all other problems.

Another use case would be non-integral cost problems. Since having non-integral capacity is problematic for this algorithm, it would make sense to separate these two type params, and use Cost type for product of them. Actually floating point types in Rust doesn't implement Ord, so..... nevermind.

[kenkoooo]

For Cap and Cost -- in theory we can use a different type for Cap, Cost and Result type where Result : From<Cap> + From<Cost>.... which can be useful for problems like this, though probably overkill for almost all other problems.

~Another use case would be non-integral cost problems. Since having non-integral capacity is problematic for this algorithm, it would make sense to separate these two type params, and use Cost type for product of them.~ Actually floating point types in Rust doesn't implement Ord, so..... nevermind.

Thanks! Let's keep using a single type parameter for now.