Change `insert` and `update` to return an enum when there are multiple unique constraints

[LHolten]

Currently the returned error type is unit (), this gives no information even though SQLite will
let us know of at least one unique constraint which failed.

[LHolten]

It is a bit unfortunate that this requires a new type in the generated API. I would much prefer that the schema macro generates an API with the least amount of types possible

[LHolten]

I kinda want to get rid of these error types altogether and replace them with a reservation based system.
The easiest way to achieve this without extra types is to limit the lifetime of each reservation to a single update or insert.
That means that it is fine to have multiple reservations of the same spot, because only one of them can actually be used in the insert or update and then the rest has their lifetime expired.
Making reservations can use the same syntax as selecting: Table.col1("test).col2("test2"). This requires a different type for this kind of expression, so technically a backwards incompatible change.
Also, schemas should be restricted to forbid overlapping unique constraints, otherwise it will be impossible to insert in tables that have them.