simplified gas accounting

[tzemanovic]

Depends on anoma/anoma#875, anoma/anoma#876, #8

For the limited set of whitelisted VPs and txs that can be used in a network, we'd like to implement a simplified gas accounting system, where each VP and tx WASM code will have (almost) constant gas consumption.

This has at least 2 high-level components:

• determining the gas consumed by a tx
• turning the gas into tx fee and charging it from the tx source account

[grarco]

Closed in favor of #1010