Remove incoming_txs and outgoing_txs from contract view of chains

This is much more realistic, as allowing contracts to efficiently access
transaction histories for all addresses is extremely expensive. To do
this, we
* Add an account_balance operation in Chain instead
* Change incoming_txs and outgoing_txs to compute transactions from
* Require implementations to give a proof-relevant trace, and rework
  proofs to use these, as necessary
3 jobs for master in 5 minutes and 58 seconds
Name Stage Failure
coq:dev Build
     = 3%Z
: Amount
Closed under the global context
make[1]: *** [all] Error 2
CoqMakefile:320: recipe for target 'all' failed
make[1]: Leaving directory '/builds/jakobbotsch/smart-contracts'
Makefile:7: recipe for target 'all' failed
make: *** [all] Error 2
ERROR: Job failed: exit code 1