breaking change -- columns identified by name rather than ordinal

[tantaman]

Closing in on a stable API but --

On main, columns are now identified by name rather than ordinal (#68) when replicating.

Reasons for this:

• ordinal/cid was found to not be stable
• this change now allows table owners to remove columns and still replicate correctly
• this change allows adding columns anywhere in the table definition and still replicate correctly
• renaming columns, however, now does require a migration step in order to update clock values to refer to the new name. This migration step will be rolled into crsql_alter_commit(table_name) and automatically applied in a future release.