Make std a facade

[brson]

Tracking rust-lang/rfcs#40

• - allow libcore to exist as just an rlib rustc: Add some support for mixing rlibs and dylibs #13892
• - bootstrap libcore with initial modules Refactor libcore out of libstd #13901
• - rustdoc inlining rustdoc: Inline documentation across crates #14391
• - migrate std::fmt Migrate std::fmt to core::fmt #14115
• - weak failure support rustc: Add official support for weak failure #14293
• - liballoc std: Refactor liballoc out of lib{std,sync} #14230
• - libcollections (underneath std, not on top of) std: Recreate a collections module  #14538
• - librustrt std: Extract librustrt out of libstd #14638
• - libsync sync: Move underneath libstd #14746