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

cranelift/fuzzgen/src/config.rs

@@ -0,0 +1,24 @@
+use std::ops::RangeInclusive;
+
+/// Holds the range of acceptable values to use during the generation of testcases
+pub struct Config {
+    pub test_case_inputs: RangeInclusive<usize>,
+    pub signature_params: RangeInclusive<usize>,
+    pub signature_rets: RangeInclusive<usize>,
+    pub instructions_per_block: RangeInclusive<usize>,
+    /// Number of variables that we allocate per function
+    /// This value does not include the signature params
+    pub vars_per_function: RangeInclusive<usize>,
+}
+
+impl Default for Config {
+    fn default() -> Self {
+        Config {
+            test_case_inputs: 1..=10,
+            signature_params: 0..=16,
+            signature_rets: 0..=16,
+            instructions_per_block: 0..=64,
+            vars_per_function: 0..=16,
+        }
+    }
+}