Support return type coercion in CREATE FUNCTION

[gorthitk]

Currently, within CREATE FUNCTION syntax, we currently restrict that the return type declared by the RETURNS clause must exactly match the expression type in the body. The change here will be relaxing that restrictions.

=== RELEASE NOTES ===

General Change
* Add support to create functions whose function body type is coercible to the declared return type.

[linux-foundation-easycla]

• ❌The commit (b7f2926e498aaf995bc308246590258b4688ded9) is missing the User's ID, preventing the EasyCLA check. Consult GitHub Help to resolve. For further assistance with EasyCLA, please submit a support request ticket.

[caithagoras]

Please squash the 3 commits into 1. You can use interactive rebase to achieve that.
https://thoughtbot.com/blog/git-interactive-rebase-squash-amend-rewriting-history

Presto Review and Commit Guidelines explains this

Avoid several substantially similar commits achieving an aggregate goal. For example several instances of “Do XYZ to class ABC”. It’s fine to do this in a pull request for review purposes, but it creates an unreadable commit history on master.

[caithagoras]

Please update the release notes section:

=== RELEASE NOTES ===

General Change
* Add support to create functions whose function body type is coercible to the declared return type.

Please also update the PR description to remove the things about block build.

[caithagoras]

lgtm % nit

[caithagoras]

Thanks for contributing! I'm merge once Travis test is passing.