feature: Add support for dumping database contents

[davidbarsky]

As discussed in this HackMD, we want to dump the contents of a Salsa database in order to debug what rust-analyzer and Salsa are doing. The original Salsa had a DebugQueryTable that we'd use like this:

let mut total_file_size = Bytes::default();
for e in ide_db::base_db::ParseQuery.in_db(db).entries::<Vec<_>>() {
    total_file_size += syntax_len(db.parse(e.key).syntax_node())
}

let mut total_macro_file_size = Bytes::default();
for e in hir::db::ParseMacroExpansionQuery.in_db(db).entries::<Vec<_>>() {
    let val = db.parse_macro_expansion(e.key).value.0;
    total_macro_file_size += syntax_len(val.syntax_node())
}
eprintln!("source files: {total_file_size}, macro files: {total_macro_file_size}");

(ParseQuery and ParseMacroExpansionQuery were generated from SourceDatabase::parse and ExpandDatabase::parse_macro_expansion, respectively.)

This PR takes a slightly different approach: instead of generating a Query struct corresponding to a tracked function or Salsa struct, this PR simply exposes the underlying pages on salsa::Storage via the debug_input_entries, debug_input_entries, and debug_interned_entries methods. This results in the following API:

let _ = InternedStruct::new(&db, "Hello, world!".to_string());

// test interned structs
let interned = db
    .storage()
    .debug_interned_entries::<InternedStruct>()
    .collect::<Vec<_>>();
    
assert_eq!(interned[0].data.0, "Hello, world!");

Few notes:

• I don't see any obvious way to make these methods generic over the underlying Salsa struct, but I'm not sure it's desirable in the first place.
• I'm not entirely happy with the name or API, so suggestions to improve it would be welcome. I don't know if I want to expose this debug information on the Salsa structs themselves, but I'd be happy to hear arguments in favor.
• This approach requires exposing parts of {interned, tracked, input}::Value publicly, which feels suboptimal, but we're all friends here, right?

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[codspeed-hq]

CodSpeed Performance Report

Merging #640 will not alter performance

_{Comparing davidbarsky:davidbarsky/add-db-debug-introspection (65f31e9) with master (88a1d77)}

Summary

✅ 9 untouched benchmarks