Make Owned empty type

[stepancheg]

Makes more clear this is marker type.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.00%. Comparing base (ab342b3) to head (e4b15a3).
⚠️ Report is 276 commits behind head on master.

Files with missing lines	Patch %	Lines
capnpc/src/codegen.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #630      +/-   ##
==========================================
- Coverage   51.64%   51.00%   -0.65%     
==========================================
  Files          69       71       +2     
  Lines       33735    32498    -1237     
==========================================
- Hits        17422    16574     -848     
+ Misses      16313    15924     -389     

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[dwrensha]

Do you ever need to construct values of this type? I was under the impression that user code would never need to do that (but it's been a while, so I could be misremembering).

[stepancheg]

Tests pass so I guess it is not meant to be constructed. (Otherwise there should be a test illustrating how construction is meant to be used.)

I am not too familiar with this library, just reading it for now and trying to make sense of it.