[prelude] add ink_prelude crate (#282)

This shall replace the ink_core::memory sub module in the future.

Cargo.toml

 [workspace]
 
 members = [
-    "utils",
+    "abi",
     "core",
-    "model",
     "lang",
     "lang2",
-    "abi",
+    "model",
+    "prelude",
+    "utils",
 ]
 exclude = [
     "examples/",

prelude/Cargo.toml

+[package]
+name = "ink_prelude"
+version = "0.1.0"
+authors = ["Robin Freyler <robin@parity.io>", "Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+license = "APACHE-2.0"
+readme = "README.md"
+
+repository = "https://github.com/paritytech/ink"
+documentation = "https://substrate.dev/substrate-contracts-workshop/#/"
+homepage = "https://www.parity.io/"
+
+description = "[ink!] Rust based eDSL for writing smart contracts for Substrate"
+keywords = ["wasm", "parity", "webassembly", "blockchain", "edsl"]
+categories = ["no-std", "embedded"]
+
+include = ["/Cargo.toml", "src/**/*.rs", "/README.md", "/LICENSE"]
+
+[lib]
+name = "ink_prelude"
+path = "lib.rs"
+
+[dependencies]
+cfg-if = "0.1"
+
+[features]
+default = ["std"]
+std = []

prelude/LICENSE

+../LICENSE
\ No newline at end of file

prelude/README.md

+../README.md
\ No newline at end of file

prelude/lib.rs

+// Copyright 2018-2019 Parity Technologies (UK) Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Data structures to operate on contract memory during contract execution.
+//!
+//! These definitions are useful since we are operating in a `no_std` environment
+//! and should be used by all ink! crates instead of directly using `std` or `alloc`
+//! crates. If needed we shall instead enhance the exposed types here.
+//!
+//! The `ink_prelude` crate guarantees a stable interface between `std` and `no_std` mode.
+
+#[cfg(not(feature = "std"))]
+extern crate alloc;
+
+use cfg_if::cfg_if;
+
+cfg_if! {
+    if #[cfg(feature = "std")] {
+        pub use std::{
+            borrow,
+            boxed,
+            format,
+            string,
+            vec,
+        };
+
+        /// Collection types.
+        pub mod collections {
+            pub use self::{
+                binary_heap::BinaryHeap,
+                btree_map::BTreeMap,
+                btree_set::BTreeSet,
+                linked_list::LinkedList,
+                vec_deque::VecDeque,
+                Bound,
+            };
+            pub use std::collections::*;
+        }
+    } else {
+        pub use alloc::{
+            borrow,
+            boxed,
+            format,
+            string,
+            vec,
+        };
+
+        /// Collection types.
+        pub mod collections {
+            pub use self::{
+                BTreeMap,
+                BTreeSet,
+                BinaryHeap,
+                LinkedList,
+                VecDeque,
+            };
+            pub use alloc::collections::*;
+            pub use core::ops::Bound;
+        }
+    }
+}