smalloc: create separate module

It will be confusing if later on we add Buffer#dispose(), and smalloc is
its own cpp api anyways. So instead create a new require('smalloc') to
expose the previous Buffer.alloc/dispose methods, and expose copyOnto
and kMaxLength as well.

Other changes:
* Added documentation and additional tests.
* smalloc::CopyOnto has changed from using assert() to throwing errors
  on bad argument values because it is not exposed to the user.
* Minor style fixes.
tools: make check-imports.sh work on bsd-likes
src: add IsFunction() assert to MakeCallback

Helps catch bugs early on. Without it, V8 throws the fairly
unhelpful exception "TypeError: undefined is not a function" -
unhelpful because there is no stack trace.
src: fix persistent handle lifecycle issue

Commit 636ca7c adds an optimization that casts strong Persistent<T>
handles directly to Local<T> handles to avoid the overhead of creating
new HandleScope-rooted Local<T> handles all the time.

One gotcha that I missed is that it's no longer legal to reference the
Local<T> after calling Persistent<T>::Dispose(). This commit addresses
that.