Use directories crate to compute config/cache directories

[soc]

Hey @oli-obk,

long time no see! :-)

As discussed, here is the reminder for using https://github.com/soc/directories-rs.

Cheers,

Simon

[oli-obk]

We're using 2.0 since a week ago or so: 724a927 and have been using directories since #543

So I guess we're all good here.

[soc]

@oli-obk Ok, great!

Though this doesn't look intended: https://github.com/rust-lang/miri/blob/master/src/bin/cargo-miri.rs#L250

I'll create a separate PR for it.

[soc]

See #774.