This CL adds support for indirect function calls to the interpreter. It
can indirectly call other wasm function in the same instance, which are
then executed in the interpreter, or call imported functions.

Implementing this required some refactoring:
- The wasm interpreter now unwraps import wrappers on demand, instead
  of unwrapping all of them on instantiation and storing a vector of
  handles. This also avoids the DeferredHandleScope completely, instead
  we just store two global handles in the code map.
- The interpreter gets the code table, function tables and signature
  tables directly from the attached wasm instance object. This ensures
  that the interpreter sees all updates to tables that might have been
  performed by external code.
- There is now common functionality for calling a code object. This is
  used for direct calls to imported functions and for all indirect
  calls. As these code objects can also be wasm functions which should
  be executed in the interpreter itself, I introduce a struct to hold
  the outcome of calling the code object, or a pointer to
  InterpreterCode to be called in the interpreter.

R=ahaas@chromium.org
BUG=v8:5822

Change-Id: I20fb2ea007e79e5fcff9afb4b1ca31739ebcb83f
Reviewed-on: https://chromium-review.googlesource.com/458417
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44059}