fuzzing: progress towards web UI

* libfuzzer: close file after mmap
* fuzzer/main.js: connect with EventSource and debug dump the messages.
  currently this prints how many fuzzer runs have been attempted to
  console.log.
* extract some `std.debug.Info` logic into `std.debug.Coverage`.
  Prepares for consolidation across multiple different executables which
  share source files, and makes it possible to send all the
  PC/SourceLocation mapping data with 4 memcpy'd arrays.
* std.Build.Fuzz:
  - spawn a thread to watch the message queue and signal event
    subscribers.
  - track coverage map data
  - respond to /events URL with EventSource messages on a timer

lib/std/Build/Step/Run.zig

@@ -1521,7 +1521,11 @@ fn evalZigTest(
                 {
                     web_server.mutex.lock();
                     defer web_server.mutex.unlock();
-                    try web_server.msg_queue.append(web_server.gpa, .{ .coverage_id = coverage_id });
+                    try web_server.msg_queue.append(web_server.gpa, .{ .coverage = .{
+                        .id = coverage_id,
+                        .run = run,
+                    } });
+                    web_server.condition.signal();
                 }
             },
             else => {}, // ignore other messages