Implement ASSERT statement

[Dandandan]

Issue: #225

See:
https://cloud.google.com/bigquery/docs/reference/standard-sql/debugging-statements
https://www.postgresql.org/docs/10/plpgsql-errors-and-messages.html

PostgreSQL supports an arbitrary expression as optional second message, so made that an expression as well.

[coveralls]

Pull Request Test Coverage Report for Build 171535046

• 35 of 38 (92.11%) changed or added relevant lines in 3 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.3%) to 91.588%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	11	12	91.67%
src/ast/mod.rs	7	9	77.78%

Files with Coverage Reduction	New Missed Lines	%
src/tokenizer.rs	1	91.68%
src/test_utils.rs	2	82.0%
src/parser.rs	4	89.02%

Totals
Change from base Build 169139020:	-0.3%
Covered Lines:	4181
Relevant Lines:	4565

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💛 - Coveralls

[nickolay]

LGTM!

[nickolay]

Do you need Box here?

[Dandandan]

Removed! Probably doesn't matter much in this case, and makes it easier to read. Would maybe be interesting in the future to see how much bytes we are using per struct (and impact on perf / memory).

[nickolay]

I assume that the only two cases when we should use Box in our AST are: cycles (e.g. Expr) and when clippy reports a large_enum_variant.

[Dandandan]

That's probably a good heuristic indeed.

[Dandandan]

Thanks!