Adding contracts for Anycol.isValueColumn etc. for smart-casting

[Jolanrensen]

Small change to reduce verbose code like:

if (col.isColumnGroup()) callFunction(col.asColumnGroup())

to

if (col.isColumnGroup()) callFunction(col)

thanks to contracts :).

Unfortunately, we cannot yet do this with enum entries, so when we're using notations like

when (col.kind) {
    ColumnKind.Group -> callFunction(col.asColumnGroup())
    ....
}

are still necessary.

[github-actions]

Generated sources will be updated after merging this PR.
Please inspect the changes in here.

[koperagen]

Cool!