Add an "umbrella" crate, which seeks to provide a convenient interface.

This currently pulls in cretonne-codegen and cretonne-frontend and
provides a simple prelude interface.

This fixes #287.

lib/umbrella/src/lib.rs

@@ -0,0 +1,19 @@
+//! Cretonne umbrella crate, providing a convenient one-line dependency.
+
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
+
+pub extern crate cretonne_codegen;
+pub extern crate cretonne_frontend;
+
+/// A prelude providing convenient access to commonly-used cretonne features. Use
+/// as `use cretonne::prelude::*`.
+pub mod prelude {
+    pub use cretonne_codegen;
+    pub use cretonne_codegen::entity::EntityRef;
+    pub use cretonne_codegen::ir::{AbiParam, InstBuilder, Value, Ebb, Signature, CallConv};
+    pub use cretonne_codegen::ir::types;
+    pub use cretonne_codegen::ir::condcodes::IntCC;
+
+    pub use cretonne_frontend::{FunctionBuilderContext, FunctionBuilder, Variable};
+}