Fix no_std error trait implementation

[elagil]

As far as I can tell, the Error trait is not fully implemented for no_std. This should fix it.

[dwrensha]

Ah, I suppose that rust-lang/rust#99917 made this possible.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 52.41%. Comparing base (ab342b3) to head (f74697c).
Report is 140 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/lib.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   51.64%   52.41%   +0.77%     
==========================================
  Files          69       70       +1     
  Lines       33735    34615     +880     
==========================================
+ Hits        17422    18144     +722     
- Misses      16313    16471     +158     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[elagil]

I guess bumping the rust version for the package would be required. Is that acceptable?

[dwrensha]

I added the "Breaking Change" label to this PR. We should include this when we bump the version to v0.21.0.

[dwrensha]

rebased onto master in 7f5c15d

[elagil]

Great!