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 配置，以及 issue 中描述的 build、clippy、rustfmt、unit、E2E 和 dynamic validator 测试阶段。跟踪当前哪些阶段会编译 CodeChain，并确定阶段 3、4 和 5 如何共享 build 输出或缓存。相关阶段能够在不丢失所述检查的情况下复用这些 artifact 或缓存，即表示完成。

由索引模型根据 Issue 内容生成。