Support entities with association column names that conflict with a field name

[kalifg]

If an entity is defined such that it has a field $a based on a column c, and an association field $b based on a column a, hydration will fail. This is because the data array that is created during hydration uses field names for keys except for associations. For associations it uses the column names, leading to a conflict.

This PR adds a prefix to the column-valued array keys to avoid clashes. The prefix uses characters that could not occur in a field name so it should be safe for all possible field and column names.

[github-actions]

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