(Budget metering) Add cost tiers to wasmi instructions

[jayz22]

Based on results from #597, we need to account for different costs of wasmi instructions in the host metering.

• To break down different wasmi instructions to several tiers, and assign a unit cost to each tier.
• Account for the unit cost in the buffered step meter.
• Expose the wasmi cost tier parameters as new Budget::CostTypes.

[jayz22]

Instruction level calibration has been done in #597.
But to add different cost tiers in wasmi, we need to evaluate wasmi's out-of-box fuel metering first -- #683

[jayz22]

After switching to wasmi fuel metering (#683), we will set these configurations to enable different instruction costs.