Array.insertM: Remove write operation

[sjakobi]

…by initializing the array with the inserted element.

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TODO:

• Check benchmarks

[treeowl]

Yes, this looks better.

[sjakobi]

According to the Core diff, this change mostly affects the union variants and some of the special inserts.

In the benchmarks it's pretty hard to detect significant changes in the noise, but it seems that I'm getting a speedup around 2% in All.HashMap.alterFInsert.ByteString and about 1% in All.HashMap.union.