Publish definition of verus! macro on crates.io

[dschoepe]

To allow verus-verified code to be used outside of verification contexts, it would be very useful to publish enough parts of vstd, builtin, and builtin_macros on crates.io to enable regular cargo builds of verified crates, even if this does not run verification. An explicit dependency on the github repository would prevent crates using the verus! macro from existing on crates.io.

While publishing verus in its entirety on crates.io is challenging, it's hopefully a simpler problem to publish just the definition of the verus! macro.

[parno]

I think we can mark this as complete at this point (modulo a bit more work to ensure regular updates).