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_errors/src/emitter.rs

@@ -17,8 +17,8 @@ use crate::styled_buffer::StyledBuffer;
 use crate::translation::{to_fluent_args, Translate};
 use crate::{
     diagnostic::DiagnosticLocation, CodeSuggestion, DiagCtxt, Diagnostic, DiagnosticMessage,
-    FluentBundle, LazyFallbackBundle, Level, MultiSpan, SubDiagnostic, SubstitutionHighlight,
-    SuggestionStyle, TerminalUrl,
+    ErrCode, FluentBundle, LazyFallbackBundle, Level, MultiSpan, SubDiagnostic,
+    SubstitutionHighlight, SuggestionStyle, TerminalUrl,
 };
 use rustc_lint_defs::pluralize;
 
@@ -1309,7 +1309,7 @@ impl HumanEmitter {
         msp: &MultiSpan,
         msgs: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<String>,
+        code: &Option<ErrCode>,
         level: &Level,
         max_line_num_len: usize,
         is_secondary: bool,
@@ -1340,9 +1340,9 @@ impl HumanEmitter {
                 buffer.append(0, "[", Style::Level(*level));
                 let code = if let TerminalUrl::Yes = self.terminal_url {
                     let path = "https://doc.rust-lang.org/error_codes";
-                    Cow::Owned(format!("\x1b]8;;{path}/{code}.html\x07{code}\x1b]8;;\x07"))
+                    format!("\x1b]8;;{path}/{code}.html\x07{code}\x1b]8;;\x07")
                 } else {
-                    Cow::Borrowed(code)
+                    code.to_string()
                 };
                 buffer.append(0, &code, Style::Level(*level));
                 buffer.append(0, "]", Style::Level(*level));
@@ -2076,7 +2076,7 @@ impl HumanEmitter {
         level: &Level,
         messages: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<String>,
+        code: &Option<ErrCode>,
         span: &MultiSpan,
         children: &[SubDiagnostic],
         suggestions: &[CodeSuggestion],