Fix build warnings

[fjarri]

Fixes #124

The problem is that various type ID/downcast-related functionality is only used in extend, which is gated behind the dev feature now. Doesn't seem to be possible to cleanly extract all of these methods into the dev module, hence they have to be gated.

I added a cargo hack run to CI to detect future occurrences of this.

[coveralls]

Pull Request Test Coverage Report for Build 17836524665

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 68.154%

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Files with Coverage Reduction	New Missed Lines	%
manul/src/session/tokio.rs	6	87.18%

Totals
Change from base Build 17680271010:	0.5%
Covered Lines:	2414
Relevant Lines:	3542

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💛 - Coveralls