Try migrating make-release.sh to a cargo xtask

[cschwan]

I revisited the point raised by @alecandido in this comment and it seems that cargo xtask could be a good fit. To summarize it in one point: instead of running ./make-release.sh X.Y.Z we'd run cargo xtask make-release X.Y.Z. The advantages are that it would run everywhere, not only where we have a shell. Maybe even more importantly, cargo xtask would also give us the opportunity to install and post-process manpages, which is a bit of a sore point right now (they're installed into a temporary directory whenever someone reads them and then they're subsequently removed).

[alecandido]

I like a lot the idea of using Rust code itself, and together with xshell (by the same author) it seems a good fit.

However, thinking about stability and support, I wonder if it's better to favor a different task runner, i.e. cargo make, which seems much more popular...
(but it wouldn't solve the language controversy...)

[cschwan]

An xtask applet has been added with commit 087b7b7.