Use ByteArray# instead of MutableByteArray# in the immutable Array

[harendra-kumar]

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Freeze it before finalizing the array.

[adithyaov]

We use MutableByteArray# as we perform a lot of immutable ops using mutable array combinators. If we use ByteArray#, we'll have to thaw-and-freeze it at a lower level, which should be fine. Is there a reason we want to use ByteArray#?