Make contract a separate runtime module (#345)

* decl_module and extract runtime mod

* Invert dependency staking←→contract

* Remove CodeOf

* Remove StorageOf and move double_map

* Comment staking test

* Clean

* Add gas_price and gas_limit

* Commit.

* Renames

* Params

* WIP

* Rename transfer to call

* WIP

* Rebuild binaries.

* WIP

* Backport ctro changes

* Call wiring

* Commit overlay.

* Rename merge → commit, into_state → ..._change_set

* WIP

* Contract creation routines

* Set code of the created account.

* Fix the ID of `create` Call

* Fix most of the warning.

* Add the simplest test in the contract crate

* Transfers work!

* Add contract_create test.

* Clean

* Add top-level create test

* Clean a bit.

* Pass gas_limit and data via create.

* Introduce OnAccountKill callback in staking

* Hook up OnAccountKill

* Comments

* Pay for gas.

* Refund unused gas in call

* Tests for zero call and zero endownment.

* Add todo about rewriting docs

* Pay for gas in create transactions

* Fix refunds

* Clean unrelevant comments

* fixup! Fix refunds

* fixup! Clean unrelevant comments

* Move DetermineContractAddress to contract

Also restore account removal test

* fixup! Clean unrelevant comments

* Inline effect_transfer, remove effect_create

Remove account_db!

* Use own new_test_ext.

* Don't account for liability

* Add some docs

* Move contract_fee into contract module

* Take GasMeter in vm::execute

* Use GasMeter throughout contract module for meter

* gas module refactoring

* Clean

* Add base call fee

* note about gas price should be taken from storage

* Add base fee for create

* Rename send → call

* Clean

* Take fee expressed in dots in gas

* Add Checked{Add,Sub,Mul,Div} to SimpleArithmetic

* Make Gas generic

* Store {call,create}_base_fee in storage

* Clean

* Rename buy_gas

* Store gas_price in the storage

* Remove unneeded comment.

* Bail out if contract already has code.

* Todos

* Refund even if top-level contract fails.

* Fix error msg

* Fix caller issue

* Extract tests module

* Add max_depth var in storage

* Remove left over gas_left

* Refactor exec

* Add test oog test.

* set_free_balance_creating

* Docs and comments.

* Update storage roots because of ContractFee move

* Rebuild binaries.

* Simplify vm code.

* Wrapping.

* Refactor a bit.

* Typo

* UpdateBalanceOutcome enum

* Style grumbles.

* Rebuild binaries.

* Always consume the given amount of gas.

* [skip ci] endownment → endowment

* Rename `AccountId` generic in on_account_kill

* Fix Cargo.lock

* Refine docs for gas meter.

* [skip ci] Add comments for gas module

* Directly assign to `return_data` at declaration

* Use slices instead of vecs to pass the input data

* Add todo about passing return data without copy

* Use checked_add instead of add with overflow

* Use return_data directly.

* Rebuild binaries.

* Rebuild binaries.