wasm: use {read, write}_unaligned methods

[folkertdev]

This is shorter and more directly expresses what we want here.

(obviously blocked on getting CI working again)

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[tgross35]

This was added in cf1dbfa when read_unaligned definitely existed, @alexcrichton any reason not to do this?

[alexcrichton]

I suspect that was just my own ignorance for not using {read,write}_unaligned, agreed this is much better!