Add the instance allocation strategy to generated fuzzing configs. (#3780)

* Add the instance allocation strategy to generated fuzzing configs.

This commit adds support for generating configs with arbitrary instance
allocation strategies.

With this, the pooling allocator will be fuzzed as part of the existing fuzz
targets.

* Refine maximum constants for arbitrary module limits.

* Add an `instantiate-many` fuzz target.

This commit adds a new `instantiate-many` fuzz target that will attempt to
instantiate and terminate modules in an arbitrary order.

It generates up to 5 modules, from which a random sequence of instances will be
created.

The primary benefactor of this fuzz target is the pooling instance allocator.

* Allow no aliasing in generated modules when using the pooling allocator.

This commit prevents aliases in the generated modules as they might count
against the configured import limits of the pooling allocator.

As the existing module linking proposal implementation will eventually be
deprecated in favor of the component model proposal, it isn't very important
that we test aliases in generated modules with the pooling allocator.

* Improve distribution of memory config in fuzzing.

The previous commit attempted to provide a 32-bit upper bound to 64-bit
arbitrary values, which skewed the distribution heavily in favor of the upper
bound.

This commit removes the constraint and instead uses arbitrary 32-bit values
that are converted to 64-bit values in the `Arbitrary` implementation.

fuzz/Cargo.toml

@@ -91,3 +91,9 @@ name = "cranelift-fuzzgen-verify"
 path = "fuzz_targets/cranelift-fuzzgen-verify.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "instantiate-many"
+path = "fuzz_targets/instantiate-many.rs"
+test = false
+doc = false