Modules now have their own local scope, represented by their own context.
Module instance objects have an accessor for every export that forwards
access to the respective slot from the module's context. (Exports that are
modules themselves, however, are simple data properties.)

All modules have a _hosting_ scope/context, which (currently) is the
(innermost) enclosing global scope. To deal with recursion, nested modules
are hosted by the same scope as global ones.

For every (global or nested) module literal, the hosting context has an
internal slot that points directly to the respective module context. This
enables quick access to (statically resolved) module members by 2-dimensional
access through the hosting context. For example,

  module A {
    let x;
    module B { let y; }
  }
  module C { let z; }

allocates contexts as follows:

[header| .A | .B | .C | A | C ]  (global)
          |    |    |
          |    |    +-- [header| z ]  (module)
          |    |
          |    +------- [header| y ]  (module)
          |
          +------------ [header| x | B ]  (module)

Here, .A, .B, .C are the internal slots pointing to the hosted module
contexts, whereas A, B, C hold the actual instance objects (note that every
module context also points to the respective instance object through its
extension slot in the header).

To deal with arbitrary recursion and aliases between modules,
they are created and initialized in several stages. Each stage applies to
all modules in the hosting global scope, including nested ones.

1. Allocate: for each module _literal_, allocate the module contexts and
   respective instance object and wire them up. This happens in the
   PushModuleContext runtime function, as generated by AllocateModules
   (invoked by VisitDeclarations in the hosting scope).

2. Bind: for each module _declaration_ (i.e. literals as well as aliases),
   assign the respective instance object to respective local variables. This
   happens in VisitModuleDeclaration, and uses the instance objects created
   in the previous stage.
   For each module _literal_, this phase also constructs a module descriptor
   for the next stage. This happens in VisitModuleLiteral.

3. Populate: invoke the DeclareModules runtime function to populate each
   _instance_ object with accessors for it exports. This is generated by
   DeclareModules (invoked by VisitDeclarations in the hosting scope again),
   and uses the descriptors generated in the previous stage.

4. Initialize: execute the module bodies (and other code) in sequence. This
   happens by the separate statements generated for module bodies. To reenter
   the module scopes properly, the parser inserted ModuleStatements.

R=mstarzinger@chromium.org,svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/11093074

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13033 ce2b1a6d-e550-0410-aec6-3dcde31c8c00