replace memmap crate

[bmc-msft]

As identified by cargo audit in our CICD, the memmap crate is apparently abandoned. We should investigate replacing it with a maintained alternative.

Reference: rustsec/advisory-db#359

[bmc-msft]

We use memmap in the coverage and debugger crates. @ranweiler or @lzybkr, since you are active in this code, can one of you investigate this?

[ranweiler]

@bmc-msft, yes. Per this discussion, the mapr crate may be a reasonable alternative. Will poke around.

[ranweiler]

At first glance: I think it would be better to push towards getting memmap itself maintained again (which is something I'd be interested in helping with). The crate itself is relatively small. mapr seems to be primarily forked so that user could land a PR for their project. The primary author of memmap has recent nontrivial GH activity. Will poke around forums.

[ranweiler]

I left a comment in the repo, volunteering to help maintain, and pinging the latest active co-maintainer, who at least has repo activity in 2020.

[ranweiler]

No response from the memmap maintainer; based on discussion, I think we should switch to memmap2-rs.