re-enable cargo miri

[nikomatsakis]

I disabled miri tests in #518 because concurrent-vec is not miri safe and filed orxfun/orx-concurrent-vec#14 to track the underlying issue. We should either figure out why concurrent-vec is not miri-safe and fix it or adopt a new library (e.g., https://github.com/droundy/append-only-vec/ would meet our relatively simple needs if they land droundy/append-only-vec#4; we could fork too).

[droundy]

I just released version 0.1.5 of append-only-vec with your PR included.

[nikomatsakis]

Thanks!

[nikomatsakis]

@droundy have you tested with miri, out of curiosity?

[davidbarsky]

Closing this, as miri is re-enabled.