Change Codec.union to not require type tag

[vlovgr]

This pull request effectively limits the use of type tags to the Codec.derive* functions.

[codecov-io]

Codecov Report

Merging #126 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #126   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         733    737    +4     
  Branches       30     28    -2     
=====================================
+ Hits          733    737    +4

Impacted Files	Coverage Δ
...les/core/src/main/scala/vulcan/internal/tags.scala	100% <ø> (ø)	⬆️
...eneric/src/main/scala/vulcan/generic/package.scala	100% <100%> (ø)	⬆️
modules/core/src/main/scala/vulcan/Codec.scala	100% <100%> (ø)	⬆️
modules/core/src/main/scala/vulcan/AvroError.scala	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 31c0236...13f8f66. Read the comment docs.