Freeze association foreign keys to reduce allocations

The string returned here will ultimately get used as a key of a hash in
the attribute set once the attributes are being built. When you give a
non-frozen string to `Hash#[]`, it will be duped. Be freezing we can
significantly reduce the number of times we end up allocating
`"user_id"`

This does not include any additional tests, as this should not have any
public facing implications. If you are mutating the result of
`Reflection#foreign_key`, please stop.

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