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/cursor.rs

@@ -19,6 +19,8 @@ pub use ir::layout::Cursor as LayoutCursor;
 /// encoding.
 pub struct FuncCursor<'f> {
     pos: CursorPosition,
+
+    /// The referenced function.
     pub func: &'f mut ir::Function,
 }
 
@@ -79,7 +81,11 @@ impl<'c, 'f> ir::InstInserterBase<'c> for &'c mut FuncCursor<'f> {
 pub struct EncCursor<'f> {
     pos: CursorPosition,
     built_inst: Option<ir::Inst>,
+
+    /// The referenced function.
     pub func: &'f mut ir::Function,
+
+    /// The target ISA that will be used to encode instructions.
     pub isa: &'f TargetIsa,
 }