Remove ndebug and replace with compiler flags (RFC 563)

[nikomatsakis]

Tracking issue for rust-lang/rfcs#563.

Nominating.

[pnkfelix]

P-backcompat-lang, 1.0 beta.

[nikomatsakis]

We have to figure out who is going to implement this.

cc @Aatch @alexcrichton @brson @huonw -- you guys seem like likely candidates and/or had strong opinions on the RFC thread, are any of you interested?

[alexcrichton]

I can implement this.