Auto merge of #2035 - alexcrichton:optimize-default, r=brson

This change mirrors the compiler by enabling optimizations by default rather
than by default producing an unoptimized Cargo build. I was curious why #2033
hit a debug assertion when we don't ship any binaries with debug assertions
enabled, but it looked like Cargo was installed via Homebrew which [does not
currently enable optimizations][homebrew]. I'll also send a PR over there, but I
figured it'd be also good to nip this in the bud and just start enabling
optimizations by default.

[homebrew]: https://github.com/Homebrew/homebrew/blob/5d09dd/Library/Formula/rust.rb#L69