introduce a web interface for fuzzing

* new .zig-cache subdirectory: 'v'
  - stores coverage information with filename of hash of PCs that want
    coverage. This hash is a hex encoding of the 64-bit coverage ID.
* build runner
  * fixed bug in file system inputs when a compile step has an
    overridden zig_lib_dir field set.
  * set some std lib options optimized for the build runner
    - no side channel mitigations
    - no Transport Layer Security
    - no crypto fork safety
  * add a --port CLI arg for choosing the port the fuzzing web interface
    listens on. it defaults to choosing a random open port.
  * introduce a web server, and serve a basic single page application
    - shares wasm code with autodocs
    - assets are created live on request, for convenient development
      experience. main.wasm is properly cached if nothing changes.
    - sources.tar comes from file system inputs (introduced with the
      `--watch` feature)
  * receives coverage ID from test runner and sends it on a thread-safe
    queue to the WebServer.
* test runner
  - takes a zig cache directory argument now, for where to put coverage
    information.
  - sends coverage ID to parent process
* fuzzer
  - puts its logs (in debug mode) in .zig-cache/tmp/libfuzzer.log
  - computes coverage_id and makes it available with
    `fuzzer_coverage_id` exported function.
  - the memory-mapped coverage file is now namespaced by the coverage id
    in hex encoding, in `.zig-cache/v`
* tokenizer
  - add a fuzz test to check that several properties are upheld

lib/docs/wasm/main.zig

@@ -53,7 +53,7 @@ export fn unpack(tar_ptr: [*]u8, tar_len: usize) void {
     const tar_bytes = tar_ptr[0..tar_len];
     //log.debug("received {d} bytes of tar file", .{tar_bytes.len});
 
-    unpack_inner(tar_bytes) catch |err| {
+    unpackInner(tar_bytes) catch |err| {
         fatal("unable to unpack tar: {s}", .{@errorName(err)});
     };
 }
@@ -750,7 +750,7 @@ export fn decl_type_html(decl_index: Decl.Index) String {
 
 const Oom = error{OutOfMemory};
 
-fn unpack_inner(tar_bytes: []u8) !void {
+fn unpackInner(tar_bytes: []u8) !void {
     var fbs = std.io.fixedBufferStream(tar_bytes);
     var file_name_buffer: [1024]u8 = undefined;
     var link_name_buffer: [1024]u8 = undefined;