Add a way to get a random u64.

This anticipates WebAssembly/wasi-random#18.

host/src/random.rs

@@ -4,10 +4,14 @@ use crate::{wasi_random, WasiCtx};
 
 #[async_trait::async_trait]
 impl wasi_random::WasiRandom for WasiCtx {
-    async fn getrandom(&mut self, len: u32) -> anyhow::Result<Vec<u8>> {
+    async fn get_random_bytes(&mut self, len: u32) -> anyhow::Result<Vec<u8>> {
         Ok((&mut self.random)
             .sample_iter(Standard)
             .take(len as usize)
             .collect())
     }
+
+    async fn get_random_u64(&mut self) -> anyhow::Result<u64> {
+        Ok((&mut self.random).sample(Standard))
+    }
 }

wit/wasi.wit

@@ -158,7 +158,16 @@ interface wasi-random {
   ///
   /// Deterministic environments must omit this function, rather than
   /// implementing it with deterministic data.
-  getrandom: func(len: u32) -> list<u8>
+  get-random-bytes: func(len: u32) -> list<u8>
+
+  /// Return a random `u64` value.
+  ///
+  /// This function must produce data from an adaquately seeded CSPRNG, so it
+  /// must not block, and the returned data is always unpredictable.
+  ///
+  /// Deterministic environments must omit this function, rather than
+  /// implementing it with deterministic data.
+  get-random-u64: func() -> u64
 
   /// A value containing 128 random bits.
   ///