Explicitly enable `bigint` benchmarks

Cargo.toml

@@ -24,6 +24,9 @@ rational = []
 bigint = ["rustc-serialize", "rand"]
 default = ["complex", "rational", "bigint"]
 
+[[bench]]
+name = "bigint"
+
 [[bench]]
 name = "shootout-pidigits"
 harness = false