napi: create napi_env as a real structure tied to the isolate

1. We define struct napi_env__ to include the isolate, the last
exception, and the info about the last error.

2. We instantiate one struct napi_env__ during module registration, but
only if there isn't one already attached to the isolate, perhaps from
the registration of a pervious module.

3. the last exception and the info about the last error are stored in
the napi_env__ struct which is associated with the isolate via
v8::Isolate::SetData().

Fixes nodejs#198
Closes nodejs#206