Fuzzer: Avoid emitting massive nested structs (#5564)

The nesting limit of around 20 was enough to cause exponential blowup. A 20K
input file lead to a 2GB wasm in one case I saw (!) which takes many seconds to
fuzz.

Instead, reduce the limit, and also check if random tells us that the random
input is done; when that's done we should stop, which limits us to O(input size).

Also do this for non-nullable types, and handle that in globals (we cannot emit a
RefAsNulNull there, so switch the global type if necessary).

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