Support user level sql function

[pythonbrad]

CREATE FUNCTION hello() AS "SELECT 'hello!';";
CREATE FUNCTION add(x int) AS "SELECT x+1;";
CREATE FUNCTION div(int, int) AS "SELECT $1/$2;";

[coveralls]

Pull Request Test Coverage Report for Build 4651078690

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 731 of 764 (95.68%) changed or added relevant lines in 24 files are covered.
• 33 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.2%) to 98.615%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/ast/mod.rs	115	116	99.14%
core/src/executor/alter/function.rs	33	35	94.29%
core/src/executor/evaluate/mod.rs	35	38	92.11%
core/src/executor/execute.rs	16	19	84.21%
core/src/data/function.rs	7	11	63.64%
core/src/store/function.rs	0	20	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/src/executor/aggregate/mod.rs	7	82.9%
core/src/executor/evaluate/mod.rs	7	97.62%
core/src/executor/execute.rs	19	91.44%

Totals
Change from base Build 4635810229:	-0.2%
Covered Lines:	35527
Relevant Lines:	36026

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💛 - Coveralls