Added ‘i128’ feature and unit test invocation

Cargo.toml

@@ -16,3 +16,4 @@ readme = "README.md"
 [features]
 default = ["std"]
 std = []
+i128 = []

ci/test_full.sh

@@ -11,3 +11,7 @@ cargo test --verbose
 # test `no_std`
 cargo build --verbose --no-default-features
 cargo test --verbose --no-default-features
+
+# test `i128`
+cargo build --verbose --features=i128
+cargo test --verbose --features=i128