add support for MERGE statement

[mobuchowski]

This PR adds support for MERGE statement based on the Snowflake syntax:
https://docs.snowflake.com/en/sql-reference/sql/merge.html

but should work with Hive based one neverthless.
https://cwiki.apache.org/confluence/display/hive/languagemanual+dml#LanguageManualDML-Merge

Signed-off-by: Maciej Obuchowski obuchowski.maciej@gmail.com

[coveralls]

Pull Request Test Coverage Report for Build 1971517694

• 166 of 196 (84.69%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 90.312%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	31	42	73.81%
src/parser.rs	46	65	70.77%

Totals
Change from base Build 1953203878:	-0.1%
Covered Lines:	7439
Relevant Lines:	8237

---

💛 - Coveralls

[alamb]

Looks good to me -- thank you @mobuchowski

[coveralls]

Pull Request Test Coverage Report for Build 1971517694

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 166 of 196 (84.69%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 90.312%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	31	42	73.81%
src/parser.rs	46	65	70.77%

Totals
Change from base Build 1953203878:	-0.1%
Covered Lines:	7439
Relevant Lines:	8237

---

💛 - Coveralls