Migrate modified Rust standard libraries to mir-json

[RyanGlScott]

Now that the mir-json-specific versions of the Rust standard libraries have been moved to the mir-json repo (in GaloisInc/mir-json#85 and GaloisInc/crucible#1319), this patch adjusts the various references to the standard libraries to ensure that SAW's documentation, CI, and test suite remain up to date.

[sauclovian-g]

You've forgotten CHANGES.md :-)

[RyanGlScott]

What would be an appropriate changelog entry? Nearly all of the changes here are on the mir-json side—aside from some minor differences in error message paths, I don't think anything else has changed that is user-visible.

[sauclovian-g]

If you've already got an rlibs setup, you'll want/need to migrate it. Pointing this out and referring to the mir-json docs is probably sufficient.

In theory people will see it in the mir-json changelog when they update mir-json, but since it affects the use of saw it's probably best to mention it in saw too.

[RyanGlScott]

Note to self: I need to bump the epoch date now that I've made some changes to the Rust tutorial.

[RyanGlScott]

The crucible-side changes landed after GaloisInc/crucible#1331, so I've pushed a separate commit that does nothing but adapt SAW to the changes from GaloisInc/crucible#1331. (See f261d7d.)