rust crates 国内镜像加速配置

rust 很不错，但是crates 经常下载有点慢，当前阿里云还没有相关的镜像，还有科大为我们提供了一个

配置方法

[registry]

index = "git://mirrors.ustc.edu.cn/crates.io-index"

说明：
如果不支持git 协议，可以使用http

index = "http://mirrors.ustc.edu.cn/crates.io-index"

cargo 0.13.0 以及高版本的，配置如下：

[source.crates-io]

registry = "https://github.com/rust-lang/crates.io-index"

replace-with = 'ustc'

[source.ustc]

registry = "git://mirrors.ustc.edu.cn/crates.io-index"

  Downloaded serde v1.0.81 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded semver-parser v0.7.0 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded syntex_syntax v0.42.0 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded term v0.4.6 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded rand_pcg v0.1.1 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded unicode-xid v0.0.3 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded regex-syntax v0.5.6 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded memoffset v0.2.1 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded syntex_pos v0.42.0 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded rand v0.5.5 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded stable_deref_trait v1.1.1 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded tokio-io v0.1.10 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded arrayvec v0.4.9 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)

  Downloaded rand_core v0.2.2 (registry `git://mirrors.ustc.edu.cn/crates.io-index`)