Somewhat recently, 'CString::from_raw' got a '#[must_use]' slapped
on it. Arguably, writing 'drop' around its return value is indeed much
clearer. So we do that here.

We also do that for 'Box::from_raw' even though it doesn't have a
'#[must_use]' on it. But the same principle applies.

PR #882

The actions-rs/toolchain we were using appears dead:
actions-rs/toolchain#216

Here's dtolnay's take: https://github.com/dtolnay/rust-toolchain

In particular, it looks like a much more maintainable path here. For
this one, if it ever goes unmaintained, I can maintain it myself without
too much fuss.

See also: https://old.reddit.com/r/rust/comments/vyx4oj/actionsrs_organization_became_unmaintained/ig54zv7/

We cheat a little bit by leveraging 'cargo add'.

Closes #889

This improvement is a consequence of the Q&A discussion from
#896

PR #897

I didn't realize this was a useful build output for a C library, but I
guess it is. Namely, it permits it to be built with other rlibs into one
giant single shared library.

Fixes #909

Closes #916