cleanup wasm const fn

[RalfJung]

We shouldn't need these feature gates any more, that was just an attempt at a transition that didn't quite work out.

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[RalfJung]

Okay the try run here confirms that this will indeed work, so -- this should be good to go. :)

[Amanieu]

Should we wait until the rustc fix lands for #1661 or is the allow still needed?

[RalfJung]

That's an unrelated problem, it would pop up in any PR. Not sure if it should block this one. The abi lint got backed out again anyway so it will be gone on the next nightly.

[RalfJung]

The new allow should be unnecessary with today's nightly, so I removed them.