Multiple statements in alter tables

Thanks to @bchelli via his PR #60 and @clemens-smartparking via this PR

🚨🚨🚨 BREAKING CHANGE 🚨🚨🚨

This is breaking the AST interface for the alter table statement

• change becomes changes (with a "s") and is an array of table alterations (TableAlteration[])

This is a PR to address the following issues:

• #57
• oguimbal/pg-mem#85
• oguimbal/pg-mem#117