Internalise typeOf and extends

[jonnyreeves]

Purpose (TL;DR) - mandatory

Moves typeOf and extends utility functions into the util/core folder so they are no longer a special case and it's more explicit that these are not part of the sinon public API.

Background (Problem in detail) - optional

Part of the sinon 2.0 roadmap, see #966.

How to verify - mandatory

• npm test
• confirmed using a built version that sinon.extend and sinon.typeOf still issue a deprecation warning.

[coveralls]

Coverage remained the same at 94.896% when pulling c8832ac on jonnyreeves:feature/internalize into 11edb2d on sinonjs:master.

[fatso83]

I had to look twice to understand why the deprecation was still printed after removing it, but I saw it in the end :-)