src: make accessors immune to context confusion

It's possible for an accessor or named interceptor to get called with
a different execution context than the one it lives in, see the test
case for an example using the debug API.

This commit fortifies against that by passing the environment as a
data property instead of looking it up through the current context.

Fixes: https://github.com/iojs/io.js/issues/1190 (again)
PR-URL: https://github.com/iojs/io.js/pull/1238


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