Cranelift CLIF-level differential fuzzer (#3038)

* cranelift: Initial fuzzer implementation

* cranelift: Generate multiple test cases in fuzzer

* cranelift: Separate function generator in fuzzer

* cranelift: Insert random instructions in fuzzer

* cranelift: Rename gen_testcase

* cranelift: Implement div for unsigned values in interpreter

* cranelift: Run all test cases in fuzzer

* cranelift: Comment options in function_runner

* cranelift: Improve fuzzgen README.md

* cranelift: Fuzzgen remove unused variable

* cranelift: Fuzzer code style fixes

Thanks! @bjorn3

* cranelift: Fix nits in CLIF fuzzer

Thanks @cfallin!

* cranelift: Implement Arbitrary for TestCase

* cranelift: Remove gen_testcase

* cranelift: Move fuzzers to wasmtime fuzz directory

* cranelift: CLIF-Fuzzer ignore tests that produce traps

* cranelift: CLIF-Fuzzer create new fuzz target to validate generated testcases

* cranelift: Store clif-fuzzer config in a separate struct

* cranelift: Generate variables upfront per function

* cranelift: Prevent publishing of fuzzgen crate

fuzz/fuzz_targets/cranelift-fuzzgen-verify.rs

@@ -0,0 +1,11 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+use cranelift_codegen::{settings, verify_function};
+use cranelift_fuzzgen::TestCase;
+
+fuzz_target!(|testcase: TestCase| {
+    let flags = settings::Flags::new(settings::builder());
+    verify_function(&testcase.func, &flags).unwrap();
+});