Add a tool to verify core/std have the same API

[alexcrichton]

In theory core is an exact subset of libstd's API surface area. In practice there's been some bugs here and there, so we should write a tool to verify that the two are the same. We can then check it into the tree and sleep soundly at night knowing that they'll never diverge again!

[alexcrichton]

cc @rust-lang/libs

[durka]

Perhaps just running libcoretest against std?

[alexcrichton]

Now that's not a bad idea! We could possibly do something like:

rustc src/libcoretest/lib.rs --extern core=path/to/libstd.rlib

[steveklabnik]

Triage: no change

there has been some discussion of eliminating the core/std distinction; if that ever comes to pass, this will end up being "fixed" by that.

[Mark-Simulacrum]

Closing as a duplicate of #60479 (which has more discussion).