make alloc and serde compilation more robust

[joseluis]

This fixes compilation that didn't succeed before like this:

cargo b --features serde,alloc --no-default-features

And by auto-enabling alloc with serde it now works also like this:

cargo b --features serde --no-default-features

And it keeps working as before, like this:

cargo b
cargo b --features serde
cargo b --no-default-features

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.35%. Comparing base (372c19b) to head (21f6f38).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/hazardous/kdf/argon2i.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
- Coverage   99.37%   99.35%   -0.02%     
==========================================
  Files         106      106              
  Lines       21677    21677              
==========================================
- Hits        21541    21538       -3     
- Misses        136      139       +3     

☔ 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.

[brycx]

Thank you, @joseluis!

This looks good to me and I don't think the change should pose any issues, especially since the serde dependency itself uses alloc when default-features are disabled.

It think we should add this to the test-suite in test.yml, to make sure this feature-combination won't regress in the future. Could I bother you to add this, @joseluis? If not that's totally fine - I'll merge this later and add it myself.

[joseluis]

I'm glad you like it. I've added it to the CI test suite.