Merge methods accessing accounts

[chfast]

Instead of

get_balance()
get_code_hash()
get_code_size()

have

struct account
{
  balance
  code_size
  code_hash
}
get_account() -> account

Replaces #117.

[axic]

Shouldn't include account_exists?

[chfast]

For non-existing accounts the get_account() should return some kind of null value.

[axic]

I see two options:

1. returns struct account, but has a field within: int exists.
2. takes pointer to struct account and returns int

I think 1) may be nicer if we want to avoid pointers.

[axic]

@chfast are you tackling this for 7?