Enable simd fuzzing on oss-fuzz (#3152)

* Enable simd fuzzing on oss-fuzz

This commit generally enables the simd feature while fuzzing, which
should affect almost all fuzzers. For fuzzers that just throw random
data at the wall and see what sticks, this means that they'll now be
able to throw simd-shaped data at the wall and have it stick. For
wasm-smith-based fuzzers this commit also updates wasm-smith to 0.6.0
which allows further configuring the `SwarmConfig` after generation,
notably allowing `instantiate-swarm` to generate modules using simd
using `wasm-smith`. This should much more reliably feed simd-related
things into the fuzzers.

Finally, this commit updates wasmtime to avoid usage of the general
`wasm_smith::Module` generator to instead use a Wasmtime-specific custom
default configuration which enables various features we have
implemented.

* Allow dummy table creation to fail

Tables might creation for imports may exceed the memory limit on the
store, which we'll want to gracefully recover from and not fail the
fuzzers.

crates/fuzzing/src/generators/table_ops.rs

@@ -4,7 +4,7 @@ use arbitrary::Arbitrary;
 use std::ops::Range;
 use wasm_encoder::{
     CodeSection, EntityType, Export, ExportSection, Function, FunctionSection, ImportSection,
-    Instruction, Limits, Module, TableSection, TableType, TypeSection, ValType,
+    Instruction, Module, TableSection, TableType, TypeSection, ValType,
 };
 
 /// A description of a Wasm module that makes a series of `externref` table
@@ -57,10 +57,8 @@ impl TableOps {
         let mut tables = TableSection::new();
         tables.table(TableType {
             element_type: ValType::ExternRef,
-            limits: Limits {
-                min: self.table_size(),
-                max: None,
-            },
+            minimum: self.table_size(),
+            maximum: None,
         });
 
         // Encode the types for all functions that we are using.