Union fixing

[ericzhang6222]

• Please attach WIP tag when this PR is still work in progress.
• Please attach breaking-change tag if this PR potentially causes other components to fail or changes previous sysl executable's behaviour.

Fixes #976 .

Changes proposed in this pull request:

• It can support following syntax:

  Test:
  !type A: ...
  !type B: ...
  !type C: ...
  
  !union U:
      A
      B
      C
  
  !union Mixed:
      int
      string
      sequence of string
      set of bytes
      A
      U

Checklist:

• [x] Added related tests
• [x] Made corresponding changes to the documentation

[codecov[bot]]

Codecov Report

Merging #1032 into master will increase coverage by 0.94%. The diff coverage is 82.06%.

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
+ Coverage   83.35%   84.29%   +0.94%     
==========================================
  Files          73       77       +4     
  Lines       10609    10713     +104     
==========================================
+ Hits         8843     9031     +188     
+ Misses       1432     1358      -74     
+ Partials      334      324      -10     

Impacted Files	Coverage Δ
pkg/database/db_test_utils.go	83.33% <0.00%> (ø)
pkg/diagrams/plantuml.go	0.00% <0.00%> (ø)
pkg/mod/fs.go	67.16% <51.61%> (-2.65%)	:arrow_down:
pkg/mod/github.go	55.85% <55.85%> (ø)
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mermaid/datamodeldiagram/datamodeldiagram.go	61.90% <68.42%> (+3.33%)	:arrow_up:
pkg/parse/parse.go	85.01% <71.87%> (+1.03%)	:arrow_up:
pkg/mod/gomodules.go	73.41% <77.77%> (ø)
pkg/mod/module.go	82.85% <79.31%> (+17.14%)	:arrow_up:
pkg/parse/fs.go	90.90% <80.00%> (-9.10%)	:arrow_down:
... and 33 more