wasmparser: fix ahash::RandomState usage in no_std mode (#1536)

* wasmparser::fix ahash::RandomState usage in no_std

The ahash::RandomState usage so far was incorrect in that it always used a new random initialization for each hash causing the same HashMap to create different hashes for the same hash inputs.

* Remove compile-time-rng feature and add Default impl

---------

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

Author: Robbepop

crates/wasmparser/src/map.rs

@@ -111,16 +111,27 @@ use std::collections::hash_map::RandomState as RandomStateImpl;
 
 // When the `std` feature is NOT active then rely on `ahash::RandomState`. That
 // relies on ASLR by default for randomness.
-#[derive(Default, Clone, Debug)]
+#[derive(Clone, Debug)]
+#[cfg(not(feature = "std"))]
+struct RandomStateImpl {
+    state: ahash::RandomState,
+}
+
 #[cfg(not(feature = "std"))]
-struct RandomStateImpl;
+impl Default for RandomStateImpl {
+    fn default() -> RandomStateImpl {
+        RandomStateImpl {
+            state: ahash::RandomState::new(),
+        }
+    }
+}
 
 #[cfg(not(feature = "std"))]
 impl BuildHasher for RandomStateImpl {
     type Hasher = ahash::AHasher;
 
     #[inline]
     fn build_hasher(&self) -> ahash::AHasher {
-        ahash::RandomState::new().build_hasher()
+        self.state.build_hasher()
     }
 }