Add arithmetic

[XIELongDragon]

Add arithmetic supported, like

goqu.C("a").Add(1) // "a" + 1
goqu.C("a").Sub(-1).As("b") // ("a" - 1) AS "b"

[codecov]

Codecov Report

Merging #324 (abd9a2e) into master (31d438d) will decrease coverage by 1.30%.
The diff coverage is 48.97%.

❗ Current head abd9a2e differs from pull request most recent head 30ad302. Consider uploading reports for the commit 30ad302 to get more accurate results

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
- Coverage   96.90%   95.59%   -1.31%     
==========================================
  Files          62       63       +1     
  Lines        3488     3586      +98     
==========================================
+ Hits         3380     3428      +48     
- Misses         92      142      +50     
  Partials       16       16              

Flag	Coverage Δ
unittests	95.59% <48.97%> (-1.31%)	⬇️

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

Impacted Files	Coverage Δ
exp/bitwise.go	91.30% <0.00%> (-8.70%)	⬇️
exp/cast.go	89.74% <0.00%> (-10.26%)	⬇️
exp/func.go	90.00% <0.00%> (-10.00%)	⬇️
exp/arithmetic.go	21.27% <21.27%> (ø)
exp/exp.go	73.40% <90.00%> (+1.97%)	⬆️
exp/ident.go	100.00% <100.00%> (ø)
exp/literal.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 @Frug can you help to take a look at this PR?