src: use STL containers instead of v8 values for static module data

Instead of putting the source code and the cache in v8::Objects,
put them in per-process std::maps. This has the following benefits:

- It's slightly lighter in weight compared to storing things on the
  v8 heap. Also it may be slightly faster since the preivous v8::Object
  is already in dictionary mode - though the difference is very small
  given the number of native modules is limited.
- The source and code cache generation templates are now much simpler
  since they just initialize static arrays and manipulate STL
  constructs.
- The static native module data can be accessed independently of any
  Environment or Isolate, and it's easy to look them up from the
  C++'s side.
- It's now impossible to mutate the source code used to compile
  native modules from the JS land since it's completely separate
  from the v8 heap. We can still get the constant strings from
  process.binding('natives') but that's all.

A few drive-by fixes:

- Re...