Actually default rustc to have debug! output

Turns out that even if the default is "enabled", that doesn't mean that the
CFG_ENABLE_DEBUG variable will be defined. Instead, test whether
CFG_DISABLE_DEBUG is defined and disable debug things if that's the case.
Add docs for extra::semver
Place *all* rustdoc output in doc/{crate}
auto merge of #9413 : bytbox/rust/master, r=alexcrichton
auto merge of #9385 : alexcrichton/rust/actually-ndebug, r=pnkfelix

Turns out that even if the default is "enabled", that doesn't mean that the
CFG_ENABLE_DEBUG variable will be defined. Instead, test whether
CFG_DISABLE_DEBUG is defined and disable debug things if that's the case.
auto merge of #9422 : alexcrichton/rust/fix-link, r=thestinger

This makes clicking on the logo do what you'd expect.
Extended the module tutorial section about files a bit
auto merge of #9398 : Kimundi/rust/work1, r=huonw