Release zstd support

[tylerchr]

In #539 and #540, @rlerdorf implemented support for zstd compression. I've found it to be effective in my built-from-source testing and would like to use it.

Obviously I can use zstd today by building from source, but I'd prefer to install from PECL and my understanding is that this would require a new release. Is that something that could be done?

I do note that @remicollet proposed cutting a 3.2.1 release in #525, but that seems not to have gotten traction and was before the zstd patches landed anyway. A new minor version may also be more appropriate here anyway given the new functionality.