circleci: Add check for "cargo +nightly fmt"

[axic]

Fixes #727.

Proof of failure: https://app.circleci.com/pipelines/github/wasmx/fizzy/7058/workflows/c5870ebb-07e4-489c-9edb-ddf976503441/jobs/64657/parallel-runs/0/steps/0-106

[codecov]

Codecov Report

Merging #834 (354e0a2) into master (2bd08ab) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 354e0a2 differs from pull request most recent head 23aec9b. Consider uploading reports for the commit 23aec9b to get more accurate results

@@            Coverage Diff             @@
##           master     #834      +/-   ##
==========================================
- Coverage   99.27%   99.26%   -0.02%     
==========================================
  Files          88       88              
  Lines       13296    13268      -28     
==========================================
- Hits        13200    13170      -30     
- Misses         96       98       +2     

Flag	Coverage Δ
rust	98.48% <ø> (ø)
spectests	?
unittests	?
unittests-32	99.32% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
bindings/rust/src/lib.rs	98.77% <ø> (ø)
test/testfloat/testfloat.cpp	95.86% <0.00%> (-1.38%)	⬇️
test/unittests/execute_floating_point_test.cpp	99.71% <0.00%> (-0.01%)	⬇️
lib/fizzy/execute.cpp	99.30% <0.00%> (-0.01%)	⬇️
test/unittests/instantiate_test.cpp	100.00% <0.00%> (ø)
test/unittests/floating_point_utils_test.cpp	100.00% <0.00%> (ø)