add support for ALTER TABLE ... RENAME COLUMN syntax

[tangwz]

What problem does this PR solve?

Fix ALTER TABLE ... RENAME COLUMN syntax

Issue: #462

MySQL Syntax:

alter_specification:
...
  | RENAME COLUMN old_col_name TO new_col_name
...

Bad SQL Case:

ALTER TABLE t RENAME COLUMN a TO b;

Check List

Tests

• Unit test

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@96e1f76). Click here to learn what that means.
The diff coverage is 42.85%.

@@            Coverage Diff            @@
##             master     #504   +/-   ##
=========================================
  Coverage          ?   71.59%           
=========================================
  Files             ?       32           
  Lines             ?     7723           
  Branches          ?        0           
=========================================
  Hits              ?     5529           
  Misses            ?     1670           
  Partials          ?      524

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)
ast/ddl.go	80.17% <42.85%> (ø)

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[kennytm]

LGTM

[tangwz]

@leoppro PTAL

[zier-one]

LGTM