Ensure topics are unique

[cmichi]

Closes #589.

[codecov-io]

Codecov Report

Merging #594 (1a23197) into master (2100fa7) will increase coverage by 12.80%.
The diff coverage is 78.57%.

@@             Coverage Diff             @@
##           master     #594       +/-   ##
===========================================
+ Coverage   67.26%   80.07%   +12.80%     
===========================================
  Files         155      156        +1     
  Lines        6925     6945       +20     
===========================================
+ Hits         4658     5561      +903     
+ Misses       2267     1384      -883     

Impacted Files	Coverage Δ
crates/lang/macro/tests/unique_topics.rs	66.66% <66.66%> (ø)
crates/env/src/engine/off_chain/db/events.rs	87.87% <100.00%> (+78.20%)	⬆️
crates/env/src/topics.rs	100.00% <100.00%> (+100.00%)	⬆️
crates/lang/codegen/src/generator/events.rs	100.00% <100.00%> (+100.00%)	⬆️
crates/storage/src/lazy/lazy_cell.rs	98.50% <0.00%> (+1.10%)	⬆️
crates/env/src/engine/off_chain/impls.rs	57.97% <0.00%> (+2.89%)	⬆️
crates/env/src/engine/off_chain/typed_encoded.rs	80.48% <0.00%> (+4.81%)	⬆️
...ates/storage/src/collections/hashmap/fuzz_tests.rs	100.00% <0.00%> (+5.47%)	⬆️
crates/env/src/api.rs	43.75% <0.00%> (+6.25%)	⬆️
... and 17 more

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[Robbepop]

All in all looks good to me!
Just some minor fixes.

[Robbepop]

LGTM