Rewrote Kernel[.#]require specs.

Notes:
* Removed the use of global variables. Use the ScratchPad facility
  provided by mspec instead.
* Used nested describe blocks to better structure the specs.
* Separated expectations about a file being loaded from expectations
  about how $LOADED_FEATURES is modified. These are distinct facets
  of behavior and only one facet of behavior should be covered in
  each example block.
* Fixed setup to clear both $LOAD_PATH and $LOADED_FEATURES so that
  the specs run in a consistent and known state. This also significantly
  simplifies expectations about $LOADED_FEATURES.
* Moved the __FILE__ and __LINE__ specs to language where they belong.
* Removed the dependency on requiring "rbconfig".

Author: Brian Ford