memfd: make "dense image" heuristic limit configurable.

[cfallin]

In #3820 we see an issue with the new heuristics that control use of
memfd: it's entirely possible for a reasonable Wasm module produced by a
snapshotting system to have a relatively sparse heap (less than 50%
filled). A system that avoids memfd because of this would have an
undesirable performance reduction on such modules.

Ultimately we should try to implement a hybrid scheme where we support
outlier/leftover initializers, but for now this PR makes the "always
allow dense" limit configurable. This way, embedders that want to ensure
that memfd is used can do so, if they have other knowledge about the
maximum heap size allowed in their system.

(Partially addresses #3820 but let's leave it open to track the hybrid
idea)

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[alexcrichton]

Seems reasonable to me! While you're at it mind updating the fuzzer to, when enabling memfd, also use some of the input fuzz data to configure this field?

[alexcrichton]

Also, while you're at it, want to bump the 1mb limit if that seems too small as a default?

[cfallin]

Done! And this is going to need a rebase over the memfd -> memory_init_cow rename so I'll wait for your changes in #3825 to land first then get this in.

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