Pricers: Re-work `instruments::options` module.

[avhz]

Backends to-do:

• Asian (with BlackScholes73)
• Bachelier
• Binary (with BlackScholes73)
• Forward start (with BlackScholes73)
• Heston
• Lookback (with BlackScholes73)
• Merton 1976
• Power (with BlackScholes73)