* [core] add Env::decode_input

* [lang/macro] remove unneeded crate dependencies

# Conflicts:
#	lang/macro/Cargo.toml

* [lang, lang/macro] add message dispatcher type code generation

* [lang/macro] add constructor dispatch enum code generation

* [lang] add executor functions and Execute trait

* [core] add decode_input to public env API

* [lang] fix incorrect trait bound in execute_message

* [lang/macro] add code generation for ink_lang::Execute

* [lang/macro] integrate Execute trait impls into contract dispatch

* [lang/macro] fix bug with constructors and messages with multiple inputs

* [lang] apply rustfmt

* [lang/macro] apply rustfmt + fix clippy warnings

* [lang] remove testable module

* [lang] clean-up lang module after implementing new dispatch codegen

# Conflicts:
#	lang/src/dispatcher.rs

* [lang] re-enable alloc init and finalize

* [lang] apply rustfmt

* [core] remove Env::input function

It has been deprecated. Users should use Env::decode_input function instead.

# Conflicts:
#	core/src/env/engine/off_chain/mod.rs

* [core, examples] fix move of CallData to env::test API

* [core] apply rustfmt

* [examples] fix examples Selector::from_str -> new

* [examples] fix multisig_plain doc-test import

* [examples] apply rustfmt to multisig_plain example

* [lang/macro] resolve some code dupes