Move the wasmtime crate directories form lib/* to wasmtime-*.

This follows a similar change to Cranelift made here:

https://github.com/CraneStation/cranelift/pull/660

wasmtime-runtime/src/sig_registry.rs

@@ -0,0 +1,41 @@
+//! Implement a registry of function signatures, for fast indirect call
+//! signature checking.
+
+use crate::vmcontext::VMSharedSignatureIndex;
+use cast;
+use cranelift_codegen::ir;
+use std::collections::{hash_map, HashMap};
+
+/// WebAssembly requires that the caller and callee signatures in an indirect
+/// call must match. To implement this efficiently, keep a registry of all
+/// signatures, shared by all instances, so that call sites can just do an
+/// index comparison.
+#[derive(Debug)]
+pub struct SignatureRegistry {
+    signature_hash: HashMap<ir::Signature, VMSharedSignatureIndex>,
+}
+
+impl SignatureRegistry {
+    /// Create a new `SignatureRegistry`.
+    pub fn new() -> Self {
+        Self {
+            signature_hash: HashMap::new(),
+        }
+    }
+
+    /// Register a signature and return its unique index.
+    pub fn register(&mut self, sig: &ir::Signature) -> VMSharedSignatureIndex {
+        let len = self.signature_hash.len();
+        match self.signature_hash.entry(sig.clone()) {
+            hash_map::Entry::Occupied(entry) => *entry.get(),
+            hash_map::Entry::Vacant(entry) => {
+                #[cfg(target_pointer_width = "32")]
+                let sig_id = VMSharedSignatureIndex::new(cast::u32(len));
+                #[cfg(target_pointer_width = "64")]
+                let sig_id = VMSharedSignatureIndex::new(cast::u32(len).unwrap());
+                entry.insert(sig_id);
+                sig_id
+            }
+        }
+    }
+}