Support PARALLEL ... and for ..ON NULL INPUT ... to CREATE FUNCTION`

[dimfeld]

Adds two sets of keywords for CREATE FUNCTION, as described at https://www.postgresql.org/docs/current/sql-createfunction.html

Parallel-save

• PARALLEL SAFE
• PARALLEL UNSAFE
• PARALLEL RESTRICTED

Called on null input

• CALLED ON NULL INPUT
• RETURNS NULL ON NULL INPUT
• STRICT

[coveralls]

Pull Request Test Coverage Report for Build 8579595348

Details

• 69 of 81 (85.19%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 87.922%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	21	33	63.64%

Totals
Change from base Build 8481586362:	0.002%
Covered Lines:	20630
Relevant Lines:	23464

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💛 - Coveralls

[alamb]

Looks good to me -- thank you for the contribution @dimfeld

[alamb]

Sorry for the delayed review. I am behind