Turn on CFG_DISABLE_DEBUG by default

[pcwalton]

There's no reason to have debug_assert! enabled for most of our binaries. This is probably a lot of the reason behind our benchmark problems!

I nominate for P-high.

[pnkfelix]

cc #17081

cc rust-lang/rfcs#563

[nikomatsakis]

cc #22492

[pnkfelix]

P-high, 1.0 polish.

[michaelwoerister]

Removed A-debuginfo. This is a build system issue.