Fix some clippy lints

Before:
```
% make lint 2>&1 | grep warnings | sort | uniq
error: could not compile `wasmer-cli` due to previous error; 25 warnings emitted
warning: `compiler-test-derive` (lib) generated 8 warnings
warning: `test-generator` (lib) generated 4 warnings
warning: `wasi-test-generator` (bin "wasi-test-generator") generated 8 warnings
warning: `wasmer-cache` (lib) generated 4 warnings
warning: `wasmer-cli` (lib) generated 25 warnings
warning: `wasmer-compiler-cranelift` (lib) generated 50 warnings
warning: `wasmer-compiler-singlepass` (lib) generated 228 warnings
warning: `wasmer-compiler` (lib) generated 3 warnings
warning: `wasmer-emscripten` (lib) generated 182 warnings
warning: `wasmer-engine-dummy` (lib) generated 6 warnings
warning: `wasmer-engine-dylib` (lib) generated 37 warnings
warning: `wasmer-engine-staticlib` (lib) generated 16 warnings
warning: `wasmer-engine-staticlib` (lib) generated 7 warnings
warning: `wasmer-engine-universal` (lib) generated 11 warnings
warning: `wasmer-engine` (lib) generated 15 warnings
warning: `wasmer-object` (lib) generated 5 warnings
warning: `wasmer-types` (lib) generated 31 warnings
warning: `wasmer-vfs` (lib) generated 4 warnings
warning: `wasmer-vm` (lib) generated 40 warnings
warning: `wasmer-wasi-experimental-io-devices` (lib) generated 11 warnings
warning: `wasmer-wasi-types` (lib) generated 3 warnings
warning: `wasmer-wasi` (lib) generated 13 warnings
warning: `wasmer-wast` (lib) generated 11 warnings
warning: `wasmer` (lib) generated 54 warnings
```

After:
```
warning: `wasmer-cli` (lib) generated 16 warnings
warning: `wasmer-compiler-cranelift` (lib) generated 48 warnings
warning: `wasmer-compiler-singlepass` (lib) generated 213 warnings
warning: `wasmer-emscripten` (lib) generated 170 warnings
warning: `wasmer-engine-dummy` (lib) generated 6 warnings
warning: `wasmer-engine-dylib` (lib) generated 4 warnings
warning: `wasmer-engine-staticlib` (lib) generated 14 warnings
warning: `wasmer-engine-staticlib` (lib) generated 5 warnings
warning: `wasmer-engine-universal` (lib) generated 9 warnings
warning: `wasmer-engine` (lib) generated 12 warnings
warning: `wasmer-object` (lib) generated 3 warnings
warning: `wasmer-types` (lib) generated 7 warnings
warning: `wasmer-vm` (lib) generated 36 warnings
warning: `wasmer-wasi-experimental-io-devices` (lib) generated 6 warnings
warning: `wasmer-wasi-types` (lib) generated 3 warnings
warning: `wasmer-wasi` (lib) generated 9 warnings
warning: `wasmer-wast` (lib) generated 9 warnings
warning: `wasmer` (lib) generated 42 warnings
```

lib/vm/src/lib.rs

@@ -13,8 +13,7 @@
         clippy::float_arithmetic,
         clippy::mut_mut,
         clippy::nonminimal_bool,
-        clippy::option_map_unwrap_or,
-        clippy::option_map_unwrap_or_else,
+        clippy::map_unwrap_or,
         clippy::print_stdout,
         clippy::unicode_not_nfc,
         clippy::use_self