[lang] Implement event syntax

[Robbepop]

Implements #62.

[codecov-io]

Codecov Report

Merging #78 into master will decrease coverage by 0.45%.
The diff coverage is 79.04%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   84.23%   83.77%   -0.46%     
==========================================
  Files          59       60       +1     
  Lines        4547     5152     +605     
==========================================
+ Hits         3830     4316     +486     
- Misses        717      836     +119

Impacted Files	Coverage Δ
lang/src/tests/mod.rs	100% <ø> (ø)	⬆️
lang/src/lib.rs	57.89% <ø> (ø)	⬆️
core/src/env/test.rs	50% <0%> (-16.67%)	⬇️
lang/src/tests/events.rs	100% <100%> (ø)
lang/src/parser.rs	97.79% <100%> (+2.07%)	⬆️
core/src/env/test_env.rs	43.64% <25%> (+2.66%)	⬆️
lang/src/ast.rs	57.72% <60.71%> (+7.72%)	⬆️
lang/src/hir.rs	81.08% <84.61%> (+2.5%)	⬆️
lang/src/gen/build.rs	85.21% <86.51%> (+5.31%)	⬆️
... and 6 more

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