Stop using String for error codes.

Error codes are integers, but `String` is used everywhere to represent
them. Gross!

This commit introduces `ErrCode`, an integral newtype for error codes,
replacing `String`. It also introduces a constant for every error code,
e.g. `E0123`, and removes the `error_code!` macro. The constants are
imported wherever used with `use rustc_errors::codes::*`.

With the old code, we have three different ways to specify an error code
at a use point:
```
error_code!(E0123)  // macro call

struct_span_code_err!(dcx, span, E0123, "msg");  // bare ident arg to macro call

\#[diag(name, code = "E0123")]  // string
struct Diag;
```

With the new code, they all use the `E0123` constant.
```
E0123  // constant

struct_span_code_err!(dcx, span, E0123, "msg");  // constant

\#[diag(name, code = E0123)]  // constant
struct Diag;
```

The commit also changes the structure of the error code definitions:
- `rustc_error_codes` now just defines a higher-order macro listing the
  used error codes and nothing else.
- Because that's now the only thing in the `rustc_error_codes` crate, I
  moved it into the `lib.rs` file and removed the `error_codes.rs` file.
- `rustc_errors` uses that macro to define everything, e.g. the error
  code constants and the `DIAGNOSTIC_TABLES`. This is in its new
  `codes.rs` file.

compiler/rustc_mir_transform/src/errors.rs

@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 
 use rustc_errors::{
-    Applicability, DecorateLint, DiagCtxt, DiagnosticArgValue, DiagnosticBuilder,
+    codes::*, Applicability, DecorateLint, DiagCtxt, DiagnosticArgValue, DiagnosticBuilder,
     DiagnosticMessage, EmissionGuarantee, IntoDiagnostic, Level,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -33,7 +33,7 @@ pub(crate) enum ConstMutate {
 }
 
 #[derive(Diagnostic)]
-#[diag(mir_transform_unaligned_packed_ref, code = "E0793")]
+#[diag(mir_transform_unaligned_packed_ref, code = E0793)]
 #[note]
 #[note(mir_transform_note_ub)]
 #[help]
@@ -66,7 +66,7 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for RequiresUnsafe {
     #[track_caller]
     fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a, G> {
         let mut diag = DiagnosticBuilder::new(dcx, level, fluent::mir_transform_requires_unsafe);
-        diag.code("E0133".to_string());
+        diag.code(E0133);
         diag.span(self.span);
         diag.span_label(self.span, self.details.label());
         let desc = dcx.eagerly_translate_to_string(self.details.label(), [].into_iter());