napi: create napi_env as a real structure (nodejs#198)

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 and
we pass it into the FunctionCallbackInfo for all subsequent entries into
N-API when we create functions/accessors/finalizers.

Once module unloading will be supported we shall have to delete the
napi_env we create during module init.

There is a clear separation between public and private API wrt. env:

1. Public APIs assert that env is not nullptr as their first action.

2. Private APIs need not validate env. They assume it's not nullptr.

Fixes nodejs#198
Closes nodejs#208