src: attach env directly to api functions

Attach the per-context execution environment directly to API functions.
Rationale:

 * Gets node one step closer to multi-isolate readiness.

 * Avoids multi-context confusion, e.g. when the caller and callee live
   in different contexts.

 * Avoids expensive calls to pthread_getspecific() on platforms where
   V8 does not know how to use the thread-local storage directly.
   (Linux, the BSDs.)

PR-URL: https://github.com/node-forward/node/pull/18


Reviewed-By: Fedor Indutny <fedor@indutny.com>