Improved cache tags DX: strings instead of nested arrays, strict validation

Before this patch landed, Drupal 8 had the following DX problems with cache tags:

1. Any code does ['foo' => TRUE'], then any other code no longer can do ['foo' => [35 => 35]]. We've had core bugs because of that.
2. If the $tags argument of drupal_merge_cache_tags() has something like ['entity_test' => [0 => '1']] and the $other argument has ['entity_test' => [1 => '1']] then the result will contain the value '1' twice.

Now, we use strings rather than arrays.

Old

$tags = array(
  'my_custom_tag' => TRUE,
  'node' => array(1, 3),
  'user' => array(7),
);

New

$tags = array(
  'my_custom_tag',
  'node:1',
  'node:3',
  'user:7',
);

API changes

1. Cache tags should be strings. An exception is now thrown when they're not, to improve the DX. This happens when using ::set() or ::setMultiple() on a cache back-end and when using Cache::invalidateTags() or Cache::deleteTags(). i.e.: always. Verified with test coverage.
2. Cache tag merging should now be done using Cache::mergeTags() rather than with NestedArray, array_merge() or drupal_merge_cache_tags().
3. Cache::buildTags() is provided for the rare use cases where you manually need to build cache tags (only 3 cases in core, but likely more in contrib).
4. HtmlViewSubscriber::convertCacheTagsToHeader() and HtmlViewSubscriber::convertHeaderToCacheTags() have been removed.

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Retroactively updated the following change records:

1. https://www.drupal.org/node/1272696
2. https://www.drupal.org/node/1534648