Add new build-in functions: GREATEST, LEST, UPPER, LOWER

[XIELongDragon]

Add new build-in functions: GREATEST, LEST, UPPER, LOWER

[codecov]

Codecov Report

Merging #314 (44c6f6d) into master (31d438d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
+ Coverage   96.90%   96.93%   +0.02%     
==========================================
  Files          62       62              
  Lines        3488     3518      +30     
==========================================
+ Hits         3380     3410      +30     
  Misses         92       92              
  Partials       16       16              

Flag	Coverage Δ
unittests	96.93% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exp/exp.go	71.42% <ø> (ø)
dialect/mysql/mysql.go	100.00% <100.00%> (ø)
dialect/sqlite3/sqlite3.go	100.00% <100.00%> (ø)
expressions.go	100.00% <100.00%> (ø)
sqlgen/expression_sql_generator.go	100.00% <100.00%> (ø)
sqlgen/sql_dialect_options.go	100.00% <100.00%> (ø)

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[XIELongDragon]

@mattn @doug-martin please help to review this PR