- Create ErrorReportingCtx and ErrorConstraintInfo, vasting reducing the
  number of arguments passed around everywhere in the error reporting code
- Create RegionErrorNamingCtx, making a given lifetime have consistent
  numbering thoughout all error messages for that MIR def.
- Make the error reporting code return the DiagnosticBuilder rather than
  directly buffer the Diagnostic. This makes it easier to modify the
  diagnostic later, e.g. to add suggestions.

Fixes some extra blank lines and makes some minor tweaks to the wording.

On obligation errors point at the unfulfilled binding when possible

CC rust-lang#42855, rust-lang#64130, rust-lang#64135.

`AdtDef` is an algebraic data type, not abstract data type

r? @varkor

Various refactorings to clean up nll diagnostics

- Create ErrorReportingCtx and ErrorConstraintInfo, vasting reducing the
  number of arguments passed around everywhere in the error reporting code
- Create RegionErrorNamingCtx, making a given lifetime have consistent
  numbering thoughout all error messages for that MIR def.
- Make the error reporting code return the DiagnosticBuilder rather than
  directly buffer the Diagnostic. This makes it easier to modify the
  diagnostic later, e.g. to add suggestions.

r? @estebank

Split out from rust-lang#58281

…r=Centril

Make some adjustments to the documentation for `std::convert::identity`

Fixes some extra blank lines and makes some minor tweaks to the wording.

update Nomicon and Reference

Cc @Centril

…r-proj-slice, r=oli-obk

Use while let slice_pattern instead of carrying an index around

r? @oli-obk