Add overlay expr

[ayushdg]

Fixes: #593
This pr adds support for the overlay syntax See https://www.postgresql.org/docs/current/functions-string.html.

• Add high level overlay expr
• Add parsing logic for overlay
• Add tests

The pr also adds a few tests cases for is not true/false and is [not] unknown added in #582

[coveralls]

Pull Request Test Coverage Report for Build 2935823468

• 58 of 66 (87.88%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 85.537%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	31	34	91.18%
src/ast/mod.rs	10	15	66.67%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	1	96.93%

Totals
Change from base Build 2889184458:	0.07%
Covered Lines:	9670
Relevant Lines:	11305

---

💛 - Coveralls

[andygrove]

LGTM. Thanks @ayushdg