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/ir/layout.rs

@@ -728,6 +728,27 @@ pub trait CursorBase {
         self
     }
 
+    /// Rebuild this cursor positioned at the bottom of `ebb`.
+    ///
+    /// This is intended to be used as a builder method:
+    ///
+    /// ```
+    /// # use cretonne::ir::{Function, Ebb, Inst};
+    /// # use cretonne::ir::layout::{Cursor, CursorBase};
+    /// fn edit_func(func: &mut Function, ebb: Ebb) {
+    ///     let mut pos = Cursor::new(&mut func.layout).at_bottom(ebb);
+    ///
+    ///     // Use `pos`...
+    /// }
+    /// ```
+    fn at_bottom(mut self, ebb: Ebb) -> Self
+    where
+        Self: Sized,
+    {
+        self.goto_bottom(ebb);
+        self
+    }
+
     /// Get the EBB corresponding to the current position.
     fn current_ebb(&self) -> Option<Ebb> {
         use self::CursorPosition::*;