smalloc: extend user API

node::Environment isn't accessible to user APIs, so extend smalloc to
also accept v8::Isolate.

Fixes: 75adde07 "src: remove `node_isolate` from source"
PR-URL: https://github.com/iojs/io.js/pull/905


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