Sharing binary on test using mutl-stage build and cache

We build CodeChain binary in every task in Travis. It takes about 10-15 mins per task.

1. To run clippy and rustfmt
2. To make sure it doesn't have a compile error.
3. Unit tests and E2E tests.
4. Dynamic validator tests 1
5. Dynamic validator tests 2.

We at least use a build output from 3 in 4, 5.
We might reuse a build cache from 2 in 3. When many features in clippy and rustfmt are stabilized, we also reuse the cache from 1 in 2.

首先定位 Travis CI 設定，以及 issue 中描述的 build、clippy、rustfmt、unit、E2E 和 dynamic validator 測試階段。追蹤目前哪些階段會編譯 CodeChain，並確定階段 3、4 和 5 如何共用 build 輸出或快取。相關階段能夠在不遺失所述檢查的情況下重複使用這些 artifact 或快取，即表示完成。