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

compiler/rustc_span/src/hygiene.rs

@@ -1163,6 +1163,8 @@ pub enum DesugaringKind {
     WhileLoop,
     /// `async Fn()` bound modifier
     BoundModifier,
+    /// Calls to contract checks (`#[requires]` to precond, `#[ensures]` to postcond)
+    Contract,
 }
 
 impl DesugaringKind {
@@ -1179,6 +1181,7 @@ impl DesugaringKind {
             DesugaringKind::ForLoop => "`for` loop",
             DesugaringKind::WhileLoop => "`while` loop",
             DesugaringKind::BoundModifier => "trait bound modifier",
+            DesugaringKind::Contract => "contract check",
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -118,6 +118,8 @@ symbols! {
         MacroRules:         "macro_rules",
         Raw:                "raw",
         Reuse:              "reuse",
+        RustcContractEnsures:  "rustc_contract_ensures",
+        RustcContractRequires: "rustc_contract_requires",
         Safe:               "safe",
         Union:              "union",
         Yeet:               "yeet",