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_lint/src/lints.rs

@@ -5,8 +5,8 @@ use std::num::NonZeroU32;
 use crate::errors::RequestedLevel;
 use crate::fluent_generated as fluent;
 use rustc_errors::{
-    AddToDiagnostic, Applicability, DecorateLint, Diagnostic, DiagnosticBuilder, DiagnosticMessage,
-    DiagnosticStyledString, SubdiagnosticMessage, SuggestionStyle,
+    codes::*, AddToDiagnostic, Applicability, DecorateLint, Diagnostic, DiagnosticBuilder,
+    DiagnosticMessage, DiagnosticStyledString, SubdiagnosticMessage, SuggestionStyle,
 };
 use rustc_hir::def_id::DefId;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
@@ -1065,7 +1065,7 @@ pub enum UnknownLintSuggestion {
 }
 
 #[derive(LintDiagnostic)]
-#[diag(lint_unknown_lint, code = "E0602")]
+#[diag(lint_unknown_lint, code = E0602)]
 pub struct UnknownLintFromCommandLine<'a> {
     pub name: String,
     #[subdiagnostic]