Add miri to CI pipeline

[Robbepop]

We only test ink_core/storage2 so far.

The miri tool requires rust-src and xargo tools to be installed.
This has been done in a separate PR already.

So far miri runs kind of successfully but it doesn't play well with RUST_WRAPPER and TARGET_DIR so we have to implement some miri specific work arounds here and there to avoid crashing other jobs as can be seen often with the clippy and test jobs.

Links: #401 (comment)

[TriplEight]

LGTM if it works

[Robbepop]

@TriplEight https://gitlab.parity.io/parity/ink/-/jobs/519777

any ideas what is causing this?

[codecov-commenter]

Codecov Report

Merging #401 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   86.27%   86.34%   +0.06%     
==========================================
  Files         131      131              
  Lines        5727     5727              
==========================================
+ Hits         4941     4945       +4     
+ Misses        786      782       -4     

Impacted Files	Coverage Δ
core/src/hash/accumulator.rs	92.59% <0.00%> (+14.81%)	⬆️

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[TriplEight]

there are some issues with miri which are suggesting me it's not yet stable enough and should be used isolated from the other CI environment:
rust-lang/miri#1311
rust-lang/miri#1421
As a matter of fact, they are proposing unsetting RUSTC_WRAPPER and CARGO_TARGET_DIR .

[Robbepop]

On hold until we fixed miri on ink_storage.

[Robbepop]

Closing this PR since it is heavily outdated. We maybe still want miri integration but I'd prefer miri to stabilize further to make it less of a pain to use for us.

[RalfJung]

Miri is used on CI by quite a few projects, as far as I know. So if you have something that you think needs to happen to make Miri more usable on CI, please let us know by reporting an issue! Right now I am not aware of any major issues.