Boolean expression alias

[XIELongDragon]

so we can support

SELECT ("a" IS NULL) AS "x", ("b" > 2) AS "y" FROM t

[XIELongDragon]

for 249

[codecov]

Codecov Report

Merging #307 (bdb1c43) into master (a7630f7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #307   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files          62       62           
  Lines        3477     3478    +1     
=======================================
+ Hits         3369     3370    +1     
  Misses         92       92           
  Partials       16       16           

Flag	Coverage Δ
unittests	96.89% <100.00%> (+<0.01%)	⬆️

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)
exp/bool.go	93.33% <100.00%> (+0.11%)	⬆️

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