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/debug.zig

@@ -19,6 +19,7 @@ pub const Dwarf = @import("debug/Dwarf.zig");
 pub const Pdb = @import("debug/Pdb.zig");
 pub const SelfInfo = @import("debug/SelfInfo.zig");
 pub const Info = @import("debug/Info.zig");
+pub const Coverage = @import("debug/Coverage.zig");
 
 /// Unresolved source locations can be represented with a single `usize` that
 /// corresponds to a virtual memory address of the program counter. Combined