message: Tweak categories based on review.
diff: @@ -4,7 +4,7 @@ description = "A collection of numeric types and traits for Rust, including bigi
  documentation = "http://rust-num.github.io/num"
  homepage = "https://github.com/rust-num/num"
  keywords = ["mathematics", "numerics", "bignum"]
  -categories = [ "algorithms", "data-structures", "science", "cryptography" ]
  +categories = [ "algorithms", "data-structures", "science" ]
  license = "MIT/Apache-2.0"
  repository = "https://github.com/rust-num/num"
  name = "num"
  