CheckTx gas estimation

[cwgoes]

Add a version of CheckTx which runs a transaction, estimates gas, and returns the required gas amount (which can then be used by clients).

Track gas through transaction execution in the Context and abort once a transaction runs out of gas.

[cwgoes]

Questions:

• Any better (less onerous) way to implement than threading errors through everything?
• We want to charge for both storage and compute, is there some known proportionality?
• Should we add a KVStoreWithGas abstraction for storage cost calculation? Would probably be the most accurate way to calculate costs, but would make the KVStore API more complex.

[cwgoes]

To-do:

• Switch to panic/recover
• Length-prefixed store values, KVStoreWithGas
• Refund unused gas

OK to calculate gas-after-read in KVstore, need to be careful about spam attacks with unbounded inputs, either size-cap or check max gas in ValidateMsg.

[cwgoes]

Closed by #963, further work outlined in #1008.