Add ExtCurrentBlockNumber in SRML::Contract (#3047)

* add block number

* add ext_current_block

* add ext_current_block test

* Update srml/contracts/src/wasm/mod.rs

Co-Authored-By: Sergei Pepyakin <[email protected]>

* Update srml/contracts/src/wasm/mod.rs

Co-Authored-By: Sergei Pepyakin <[email protected]>

* change valid comment

* use BlockNumberOf<T>

* Update srml/contracts/src/exec.rs

Co-Authored-By: André Silva <[email protected]>

* return types copy, not reference

* version up spec_version and impl_version

* change method name current_block -> block_number

* Update srml/contracts/src/exec.rs

Co-Authored-By: Sergei Pepyakin <[email protected]>