* [ink] Update rust-toolchain from 2019-04-20 to 2019-05-21

* [core,lang] Update & fix Cargo information

* [chores] Update rustfmt version to the current toolchain

* [core] Clean up, fix some warnings

* [core] Remove unneeded dependency

* [core] Ignore return value of Key ops

Fixes a warning

* [core,model] Adjust warning level

* [lang] Minor rustfmt run

* [ink] Add topics to events

For now, ink! will simply leave topics slice empty.

* [core] Add ext_gas_left, ext_gas_price and ext_value_transferred bindings

* [core] Clean up some code by dedup

* [core] Further code clean up

* [core] Simplify SrmlEnv::load impl