Match MySQL's handling of nested subroutine definitions.

[nicktobey]

A "subroutine" refers to any statement or block of statements that is defined but may not be immediately executed. MySQL has six types of subroutines:

• CREATE PROCEDURE
• CREATE FUNCTION
• CREATE TRIGGER
• CREATE EVENT
• CREATE TABLE AS
• CREATE VIEW AS

Dolt has all of these except functions.

Nesting these constructs is allowed in some cases but not others. Prior to fixing #8028, attempting to nest these constructs caused a panic. Now, while there is no panic, our behavior does not necessarily match MySQL

• In cases where both Dolt and MySQL return an error, Dolt's error message should match MySQL.

• In cases where Dolt returns an error but MySQL doesn't, we should match MySQL's behavior.

• In cases where MySQL returns an error but Dolt doesn't, we should either match MySQL, or add additional tests to verify that our behavior is sensible.