[Rust] Improving lengthy build times in Appveyor

[asfimport]

The Rust build is the longest-running build in Appveyor now by 7-10 minutes, perhaps inflated more now because of the Parquet donation

https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/21118767

Would it be possible to split this into two builds, or otherwise improve the runtime?

Reporter: Wes McKinney / @wesm
Assignee: Chao Sun / @sunchao

PRs and other links:

• GitHub Pull Request #3231

_{Note: This issue was originally created as ARROW-4080. Please see the migration documentation for further details.}

[asfimport]

Chao Sun / @sunchao:
Let me see if we can do improve this. Seems travis CI is also pretty long.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3231
#3231