Third experimental release

Since last release: in the src directory alone, there were 83 files changed, 6225 lines added, 4343 lines removed.

Code has gotten clearer, safer (thanks @ossfuzz!), and more efficient all around. Of note: a lot of memory allocations in the fast path are gone; deferred callbacks for coroutines are really cheap now; parsers (HTTP, configuration, template) are much more robust, coroutine context switch is pretty decent (~22ns on a SNB processor), zstd+brotli support, event loop has been completely overhauled, a few tweaks in the template engine, etc; the list goes on.

A lot of changes were covered in details in this blog post.