Update to cranelift 0.20.0.

The biggest change is the split from FunctionIndex to
DefinedFuncIndex to FuncIndex. Take better advantage of this by
converting several Vecs to PrimaryMaps.

Also, table_addr can now handle indices of the table index type,
so we don't need to explicitly uextend them anymore.

lib/execute/src/lib.rs

@@ -13,6 +13,7 @@
 )]
 
 extern crate cranelift_codegen;
+extern crate cranelift_entity;
 extern crate cranelift_wasm;
 extern crate memmap;
 extern crate region;