Tweak categories based on review.

Cargo.toml

@@ -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"