Auto merge of #3682 - alexcrichton:serde, r=brson

Migrate from rustc-serialize to Serde

This commit migrates Cargo as much as possible from rustc-serialize to
Serde. This not only provides an excellent testing ground for the toml
0.3 release but it also is a big boost to the speed of parsing the JSON
bits of the registry.

This doesn't completely excise the dependency just yet as docopt still
requires it along with handlebars. I'm sure though that in time those
crates will migrate to serde!