Add support for CREATE TYPE (AS) statements

[srijs]

Hi,

This patch adds support for parsing CREATE TYPE <name> AS (...) (https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#_11_51_user_defined_type_definition) statements. I tried my best to align the naming here with the SQL 2016 standard.

I've already introduced an enum that could be extended later to cover other types of CREATE TYPE statements, but that is not done here yet mostly because I only require the support for composite types at the moment, and the other statement types are a bit more complex to parse.

Any feedback welcome!

[coveralls]

Pull Request Test Coverage Report for Build 5210193448

• 48 of 56 (85.71%) changed or added relevant lines in 4 files are covered.
• 148 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.7%) to 87.005%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/ddl.rs	8	9	88.89%
src/ast/mod.rs	3	5	60.0%
src/parser.rs	23	28	82.14%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	148	11.94%

Totals
Change from base Build 5073704019:	0.7%
Covered Lines:	14870
Relevant Lines:	17091

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💛 - Coveralls

[alamb]

Looks good to me -- thank you @srijs

[alamb]

I took the liberty of merging up from main and fixing the doc comment, and plan to merge this PR when the CI passes

[srijs]

Thank you @alamb!

[alamb]

I plan a new release in a week or two: #887