* Checks are now split in two modules: checks.cons for consistency
checks, checks.insn for instructions.
* Checks are derived from PowerFVCheck and have a shorthand (e.g.
"insn_b"). PowerFVCheck holds a mapping between its subclasses and
their shorthands.
* Instruction checks definitions have been simplified to one-liners,
and grouped into a single file.
* A Trigger class has been added to define testbench triggers.
Jean-François Nguyen