Move WasmRuntime::translate_call_indirect() into FuncEnvironment.

Add two new arguments:

- table_index is the WebAssembly table referenced in the indirect call.
- sig_index is the WebAssembly signature index. We still have the SigRef
  that was created by make_indirect_sig(), but the WebAssembly signature
  index may be needed for detecting type mismatches at runtime.

Change the insertion location to a plain FuncCursor rather than a
FunctionBuilder<Local>. The fact that cretonne-wasm uses FunctionBuilder
should be an implementation detail, and the callbacks don't need to
access WebAssembly locals, so they don't need the extended interface.

Add a FunctionBuilder::cursor() method which creates a FuncCursor for
inserting instructions in the current EBB.

Also add a FuncEnvironment::translate_call() method which allows the
environment to override direct calls the same way as indirect calls.

lib/cretonne/src/lib.rs

@@ -17,6 +17,7 @@ pub mod entity;
 
 pub mod binemit;
 pub mod bitset;
+pub mod cursor;
 pub mod dominator_tree;
 pub mod flowgraph;
 pub mod ir;
@@ -31,7 +32,6 @@ pub mod verifier;
 mod abi;
 mod constant_hash;
 mod context;
-mod cursor;
 mod iterators;
 mod legalizer;
 mod licm;