Add a fuzz mode to stress unaligned wasm addresses (#3516)

Alignment on all memory instructions in wasm is currently best-effort
and not actually required, meaning that whatever wasm actually uses as
an address should work regardless of whether the address is aligned or
not. This is theoretically tested in the fuzzers via
wasm-smith-generated code, but wasm-smith doesn't today have too too
high of a chance of generating an actual successful load/store.

This commit adds a new configuration option to the `Config` generator
for fuzzing which forces usage of a custom linear memory implementation
which is backed by Rust's `Vec<u8>` and forces the base address of
linear memory to be off-by-one relative to the base address of the
`Vec<u8>` itself. This should theoretically force host addresses to
almost always be unaligned, even if wasm addresses are otherwise
aligned.

The main interesting fuzz coverage here is likely to be in the existing
`differential` target which compares running the same module in wasmtime
with two different `Config` values to ensure the same results are
produced. This probably won't increase coverage all that much in the
near future due to wasm-smith rarely generating successful loads/stores,
but in the meantime by hooking this up into `Config` it also means that
we'll be running in comparison against v8 and also ensuring that all
spec tests succeed if misalignment is forced at the hardware level.

As a side effect this commit also cleans up the fuzzers slightly:

* The `DifferentialConfig` struct is removed and folded into `Config`
* The `init_hang_limit` processing is removed since we don't use
  `-ttf`-generated modules from binaryen any more.
* Traps are now asserted to have the same trap code, otherwise
  differential fuzzing fails.
* Some more debug logging was added to the differential fuzzer

fuzz/fuzz_targets/differential.rs

@@ -4,8 +4,8 @@ use libfuzzer_sys::fuzz_target;
 use wasmtime_fuzzing::{generators, oracles};
 
 fuzz_target!(|data: (
-    generators::DifferentialConfig,
-    generators::DifferentialConfig,
+    generators::Config,
+    generators::Config,
     generators::GeneratedModule,
 )| {
     let (lhs, rhs, mut wasm) = data;