union: support occupying horizontal multiple fields for list field

[Kybxd]

• If a union field has property: cross:x(x not zero), it occupies x fields of this union. For example, if a union's Field2 has type []int64|{cross:3}, it occupies 3 fields (Field2, Field3, Field4), and if a next field exists, it should be defined at Field5.
• If a union field has property: cross:-1, it occupies all following fields of this union. No next field should exist after a cross:-1 field.

• close union: if the last field of a union is an incell list, extend it just like a horizontal list #171
• table parser testing
• confgen table parser: encaplulate standalone functions to parse map/list at different layouts for code reuse
• protogen: document parser not supported

[codecov]

Codecov Report

Attention: Patch coverage is 75.09091% with 137 lines in your changes missing coverage. Please review.

Project coverage is 68.76%. Comparing base (cd9e3b1) to head (91af022).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/confgen/table_parser.go	76.90%	54 Missing and 25 partials ⚠️
internal/confgen/document_parser.go	55.12%	27 Missing and 8 partials ⚠️
internal/confgen/parser.go	80.28%	8 Missing and 6 partials ⚠️
internal/protogen/protogen.go	66.66%	3 Missing and 3 partials ⚠️
log/logger.go	60.00%	2 Missing ⚠️
internal/protogen/exporter.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   68.01%   68.76%   +0.74%     
==========================================
  Files          76       76              
  Lines        9699     9649      -50     
==========================================
+ Hits         6597     6635      +38     
+ Misses       2511     2444      -67     
+ Partials      591      570      -21     

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