Enhance ecp and pk module (#344)

* add functionalities
- Add a function to access `Z` field of `EcPoint`.
- Add a newer version of `EcPoint::mul` with RNG for blinding.
  - Also marked old version one as deprecated.
- Add a newer version of `Pk::private_from_ec_components` with RNG for blinding.
  - Also marked old version one as deprecated.
- Update tests to use above new functions.
- Add a function for performing const time comparison of `EcPoint`.

* bump mbedtls version to 0.12.3

* add test for fn `eq_const_time`

* add bench to ensure eq_const_time works

* add cargo bench to CI

.github/workflows/test.yml

@@ -117,6 +117,18 @@ jobs:
           cargo fmt --check -p mbedtls
           cargo fmt --check -p mbedtls-platform-support
           cargo fmt --check -p mbedtls-sys-auto
+
+  bench:
+    name: Cargo bench
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: stable
+      - name: Run cargo bench
+        run: cargo bench
+
   ci-success:
     name: ci
     if: always()