Add EntityClass#test(id) and support update { it[nullable] = nullable }

[vlsi]

Sorry for 7 commits in a single PR, however, I expect they could just land to the main branch as is, and creating 7 different PR and rebasing them 6 times (after each gets merged) would be unnecessary overhead (for both of you and me).

Notes:

1. I adjusted generics in UpdateBuilder, so there's one type parameter and others are moved to function arguments. That makes exceptions slightly easier to read and understand.

2. I would like to reorder set functions in UpdateBuilder: first put 2 setters for regular column type, then 2 setters for EntityID column. However, I did not do that because it would make review/comparison harder. I can do that in a separate commit if you like.

3. it[nullableRef] = null does not compile now. I have no idea how to solve it as it can't decide between null as Int? and null as EntityID<Int>?. The workaround is to use null as EntityID<Int>?. It might make sense to add UpdateBuilder#setNull(Column) API so users could write:

   fun <T> setNull(column: Column<T?>) = set(column, null as T?)
   
   update {
       it.setNull(y1) // instead of it[y1] = null
   }
   

Fixes #1275, #1276, #1268

[vlsi]

This is the key signature change to support it[nullableIntRef] = null as Int?. The rest (e.g. out) is done to keep existing tests pass, and make code consistent.