fix: schemas/util.sql: remake_diff_table(): util.diff() call: include explicit types on arguments to ensure that the correct diff() variant is used

schemas/util.sql: added has_single_row()

schemas/vegbien.sql: remake_diff_table(): prepend ~ to the type table so that it sorts at the end, away from the main tables

bugfix: schemas/vegbien.sql: remake_diff_table(): can't have in_table/out_table inherit from each other, because inheritance also causes the rows of the parent table to be included in the child table. instead, they need to inherit from a common, empty table, as implemented by the new util.remake_diff_table().

schemas/util.sql: added remake_diff_table()

schemas/util.sql: drop_*(): added force param to use CASCADE mode

fix: schemas/util.sql: diff(regclass, regclass): usage: col_type_null must actually be a shared base type of the tables

fix: schemas/util.sql: diff(regclass, regclass): usage: text arguments must be cast to regclass to bind the right variant of diff()

fix: schemas/util.sql: diff(regclass, regclass): renamed params to *_table because these actually can't be views, since views don't support inheritance

schemas/util.sql: added copy_struct()