sqlparser: add support for desc/describle/explain table [col|wild] #700

[hustjieke]

[summary] add support for describle stmt. [test case] src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/explain_test.go src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/parse_test.go

[patch codecov] src/proxy/show.go 90.0% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast.go 96.6%

[codecov[bot]]

Codecov Report

Merging #704 (a25a7b3) into master (4eb64ff) will decrease coverage by 0%. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #704   +/-   ##
=====================================
- Coverage      89%    88%   -1%     
=====================================
  Files         130    130           
  Lines        9281   9277    -4     
=====================================
- Hits         8269   8252   -17     
- Misses        670    680   +10     
- Partials      342    345    +3     

Impacted Files	Coverage Δ
src/proxy/show.go	82% <100%> (-1%)	:arrow_down:
src/proxy/explain.go	63% <0%> (-20%)	:arrow_down:
src/backend/xacheck.go	80% <0%> (-2%)	:arrow_down:
src/backend/txn.go	86% <0%> (-1%)	:arrow_down:

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[hustjieke]

Incomplete comments， lack of [summary]

Complete.

[zhyass]

LGTM