Implement ON CONFLICT and RETURNING

[main--]

Implement the ON CONFLICT and RETURNING extensions as used in e.g. PostgreSQL and Sqlite.

Fixes #60.

[alamb]

CI is failing on this PR. Marking as draft

[coveralls]

Pull Request Test Coverage Report for Build 3444008600

• 117 of 126 (92.86%) changed or added relevant lines in 5 files are covered.
• 1023 unchanged lines in 17 files lost coverage.
• Overall coverage increased (+0.5%) to 86.283%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	24	25	96.0%
src/ast/mod.rs	25	33	75.76%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	2	10.0%
src/test_utils.rs	2	83.58%
tests/sqlparser_bigquery.rs	6	94.29%
tests/sqlparser_sqlite.rs	6	94.87%
src/ast/value.rs	8	87.67%
tests/sqlparser_clickhouse.rs	8	95.09%
tests/sqlparser_hive.rs	8	96.54%
tests/sqlparser_mssql.rs	8	94.7%
tests/sqlparser_redshift.rs	9	93.57%
tests/sqlparser_snowflake.rs	9	93.67%

Totals
Change from base Build 3209998484:	0.5%
Covered Lines:	11675
Relevant Lines:	13531

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💛 - Coveralls

[alamb]

Thank you for the contribution @main-- Can you please add tests to this PR?

[baoyachi]

Is there any update 🦀 @alamb

[main--]

@baoyachi I think right now @alamb is waiting for me to write tests that make sure that the new functionality actually works. I don't have the bandwidth to work on this right now, however.

If you're interested in moving this PR forward, you could write some tests and I would be happy to merge them onto my branch to make them part of this PR. :)

[alamb]

Yes, I think this PR needs tests before it can be merged. Also I plan to invest some time today working down the outstanding sqlparser-rs review backlog. FYI @AugustoFKL

[alamb]

Looks great to me -- thank you @main--