Support CREATE FUNCTION for BigQuery

[iffyio]

Adds support for the CREATE FUNCTION statement in BigQuery. Merges with the representation used in the hive/postgres dialects.

https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#create_function_statement

[coveralls]

Pull Request Test Coverage Report for Build 9302309555

Details

• 228 of 239 (95.4%) changed or added relevant lines in 5 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 89.502%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_bigquery.rs	34	36	94.44%
src/parser/mod.rs	141	145	97.24%
src/ast/mod.rs	36	41	87.8%

Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	80.92%
src/parser/mod.rs	3	92.98%

Totals
Change from base Build 9212358328:	0.03%
Covered Lines:	25049
Relevant Lines:	27987

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💛 - Coveralls

[jmhain]

lgtm! cc @alamb

[alamb]

Thank you @iffyio and @jmhain