Express contracts as part of function header and lower it to the contract lang items

includes post-developed commit: do not suggest internal-only keywords as corrections to parse failures.

includes post-developed commit: removed tabs that creeped in into rustfmt tool source code.

includes post-developed commit, placating rustfmt self dogfooding.

includes post-developed commit: add backquotes to prevent markdown checking from trying to treat an attr as a markdown hyperlink/

includes post-developed commit: fix lowering to keep contracts from being erroneously inherited by nested bodies (like closures).

Rebase Conflicts:
 - compiler/rustc_parse/src/parser/diagnostics.rs
 - compiler/rustc_parse/src/parser/item.rs
 - compiler/rustc_span/src/hygiene.rs

Remove contracts keywords from diagnostic messages

library/core/src/lib.rs

@@ -113,8 +113,8 @@
 #![feature(bigint_helper_methods)]
 #![feature(bstr)]
 #![feature(bstr_internals)]
-#![feature(const_carrying_mul_add)]
 #![feature(closure_track_caller)]
+#![feature(const_carrying_mul_add)]
 #![feature(const_eval_select)]
 #![feature(core_intrinsics)]
 #![feature(coverage_attribute)]