Add examples for hack lang (joewalnes#337)

Thanks @azjezz and @usox

examples/hack/README.md

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+This examples directory shows some examples written in [Hack](https://hacklang.org).
+
+### Requirements :
+
+- [HHVM](https://github.com/facebook/hhvm) 3.30+
+- [HSL ( Hack Standard Library )](https://github.com/hhvm/hsl) 3.30+
+- [HSL Experimental](https://github.com/hhvm/hsl-experimental) 3.30+
+
+You can also test the command files by running from the command line :
+
+```
+$ hhvm count.hh
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+```

examples/hack/count.hh

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+#!/usr/bin/hhvm
+<?hh // strict
+
+use namespace HH\Lib\Str;
+use function HH\Lib\Experimental\IO\request_output;
+use function usleep;
+
+// Simple example script that counts to 10 at ~2Hz, then stops.
+
+<<__EntryPoint>>
+async function count_to_ten(): Awaitable<noreturn> {
+  $output = request_output();
+  for ($count = 1; $count <= 10; $count++) {
+    await $output->writeAsync(
+      Str\format("%d\n",$count)
+    );
+
+    // usleep is builtin, it is not an async builtin - so it also must block the main request thread
+    usleep(500000);
+  }
+
+  // flush output
+  await $output->flushAsync();
+
+  exit(0);
+}

examples/hack/dump-env.hh

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+#!/usr/bin/hhvm
+<?hh // strict
+
+use namespace HH\Lib\Str;
+use function HH\Lib\Experimental\IO\request_output;
+
+<<__EntryPoint>>
+async function dumpEnv(): Awaitable<noreturn> {
+  // Standard CGI(ish) environment variables, as defined in
+  // http://tools.ietf.org/html/rfc3875
+  $names = keyset[
+    'AUTH_TYPE',
+    'CONTENT_LENGTH',
+    'CONTENT_TYPE',
+    'GATEWAY_INTERFACE',
+    'PATH_INFO',
+    'PATH_TRANSLATED',
+    'QUERY_STRING',
+    'REMOTE_ADDR',
+    'REMOTE_HOST',
+    'REMOTE_IDENT',
+    'REMOTE_PORT',
+    'REMOTE_USER',
+    'REQUEST_METHOD',
+    'REQUEST_URI',
+    'SCRIPT_NAME',
+    'SERVER_NAME',
+    'SERVER_PORT',
+    'SERVER_PROTOCOL',
+    'SERVER_SOFTWARE',
+    'UNIQUE_ID',
+    'HTTPS'
+  ];
+
+  /* HH_IGNORE_ERROR[2050] using global variable */
+  $server = dict($_SERVER);
+
+  $ouput = request_output();
+
+  foreach($names as $name) {
+    await $output->writeAsync(
+      Str\format("%s = %s\n", $name, $server[$name] ?? '<unset>')
+    );
+  }
+
+  // Additional HTTP headers
+  foreach($server as $k => $v) {
+     if ($k is string && Str\starts_with($k, 'HTTP_')) {
+        await $output->writeAsync(
+          Str\format("%s = %s\n", $k, $v as string)
+        );
+     }
+  }
+
+  // flush output
+  await $output->flushAsync();
+
+  exit(0);
+}

examples/hack/greeter.hh

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+#!/usr/bin/hhvm
+<?hh // strict
+
+use namespace HH\Lib\Str;
+use namespace HH\Lib\Experimental\IO;
+
+<<__EntryPoint>>
+async function greeter(): Awaitable<noreturn> {
+  // For each line FOO received on STDIN, respond with "Hello FOO!".
+  $input = IO\request_input();
+  $output = IO\request_output();
+  while(!$input->isEndOfFile()) {
+    await $ouput->writeAsync(
+      Str\format("Hello %s!\n", await $input->readLineAsync())
+    );
+  }
+
+  // flush output
+  await $output->flushAsync();
+
+  exit(0);
+}