Add branch_if_overflow_set_test().
Add branch_if_overflow_set().
Add BVC to execute_instruction().
Merge pull request #29 from amw-zero/ark-bvs

Implement BVS instruction
Add compare_test().
Add compare().  Add more cases to compare_test().
Add CMP to execute_instruction().
Make compare() take a variable register.
Add CPX and CPY.
Merge pull request #30 from amw-zero/ark-cmp-cpx-cpy

Implement CMP, CPX, and CPY instructions
Add exclusive_or_test().
Add EOR to execute_instruction().
Add exclusive_or().
Merge pull request #31 from amw-zero/ark-eor

Implement EOR