Window function support

[doug-martin]

• [ADDED] Window Function support Added Window Function support #128

[codecov]

Codecov Report

Merging #148 into master will increase coverage by 1.96%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   91.86%   93.82%   +1.96%     
==========================================
  Files          53       55       +2     
  Lines        3589     3823     +234     
==========================================
+ Hits         3297     3587     +290     
+ Misses        273      217      -56     
  Partials       19       19

Impacted Files	Coverage Δ
exp/exp.go	68.49% <ø> (ø)	⬆️
exp/select_clauses.go	95.6% <100%> (+0.39%)	⬆️
exp/col.go	86.66% <100%> (+11.11%)	⬆️
exp/window.go	100% <100%> (ø)
sqlgen/sql_dialect_options.go	100% <100%> (+18.75%)	⬆️
sqlgen/select_sql_generator.go	100% <100%> (ø)	⬆️
dialect/sqlite3/sqlite3.go	100% <100%> (ø)	⬆️
exp/func.go	33.33% <100%> (+14.58%)	⬆️
exp/window_func.go	100% <100%> (ø)
select_dataset.go	100% <100%> (ø)	⬆️
... and 11 more

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[doug-martin]

@Xuyuanp thank you for the pull request and including all the tests and examples great job!

I had to make a few changes to make the expressions and sql generation work. The most notable ones were:

• Renamed SelectDataset#Windows to SelectDataset#Window to keep inline with the other naming conventions such as Select and Where
• Moving the Over and OverName from SQLWindowFunctionExpression onto SQLFunctionExpression...after doing some research I discovered that you can use the window functions on most function so it made sense to move that to the normal SQLFunctionExpression.
• I made most of the Window expressions use IdentifierExpressions instead of strings.
• Moved the window sql generation to the new sql package.
• Added unit tests for sql generation on all the new expression types.

[doug-martin]

I also want to note that I ended up going with the snake case function names like you suggested, It does read better and keeps with the already established function naming conventions.

[Xuyuanp]

@doug-martin goqu is the best 'sql builder' lib I have found and I have used it in my project.
It's my honor to make it better.