Switch defaults from generating unstable Rust to generating stable Rust

[fitzgen]

The current default is definitely the wrong choice.

This involves

• changing the Builder::no_unstable_rust method to the Builder::unstable_rust method

• changing the --no-unstable-rust flag to a --unstable-rust flag in src/options.rs

• I don't think this will require updating test results since we run all the tests with --no-unstable-rust, however it may require removing that flag from the test harness. I sugget git greping for "unstable" to see places in the code base that may require updating.

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[jonstites]

I'll take a stab at this one. @highfive: assign me

[highfive]

Hey @jonstites! Thanks for your interest in working on this issue. It's now assigned to you!

[rigelk]

@jonstites sorry, didn't see you had taken it :/