Removed dependency to "rustfmt" from intercom-attributes

[Fluxie]

Rustftm is sometimes out-of-sync with nightly. This caused build problems.

Now the test in "intercom-attributes" create a temporary crate from the source file that needs to be formatted. Then "cargo fmt" is executed in order to format the source files.

[codecov-io]

Codecov Report

Merging #66 into master will decrease coverage by 1.68%. The diff coverage is 46.1%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   76.47%   74.78%   -1.69%     
==========================================
  Files          52       52              
  Lines        3213     3320     +107     
==========================================
+ Hits         2457     2483      +26     
- Misses        756      837      +81

Impacted Files	Coverage Δ
intercom-common/src/foreign_ty.rs	100% <ø> (ø)	:arrow_up:
intercom-common/src/builtin_model.rs	91.3% <ø> (ø)	:arrow_up:
intercom-common/src/idents.rs	100% <ø> (ø)	:arrow_up:
intercom-common/src/attributes/com_library.rs	0% <0%> (ø)	:arrow_up:
intercom-fmt/src/main.rs	0% <0%> (ø)
intercom-common/src/generators/idl.rs	82.11% <100%> (ø)	:arrow_up:
intercom-common/src/methodinfo.rs	86.36% <100%> (ø)	:arrow_up:
intercom-common/src/generators/cpp.rs	80.59% <100%> (ø)	:arrow_up:
intercom-common/src/attributes/com_impl.rs	98.24% <100%> (ø)	:arrow_up:
intercom-common/src/attributes/com_interface.rs	98.68% <100%> (ø)	:arrow_up:
... and 16 more

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[Rantanen]

It might be worth it to break the intercom-fmt into an independent crate at some point. It feels like the functionality provided by that crate might be useful for other projects as well.

I'll merge this in now since we need it badly now. :)