Add a compilation context struct.

This will provide main entry points for compiling functions, and it
serves as a place for keeping data structures that should be preserved
between function compilations to reduce allocator thrashing.

So far, Context is just basic scaffolding. More to be added.

lib/cretonne/src/context.rs

@@ -0,0 +1,23 @@
+//! Cretonne compilation context and main entry point.
+//!
+//! When compiling many small functions, it is important to avoid repeatedly allocating and
+//! deallocating the data structures needed for compilation. The `Context` struct is used to hold
+//! on to memory allocations between function compilations.
+
+use ir::Function;
+
+/// Persistent data structures and compilation pipeline.
+pub struct Context {
+    /// The function we're compiling.
+    pub func: Function,
+}
+
+impl Context {
+    /// Allocate a new compilation context.
+    ///
+    /// The returned instance should be reused for compiling multiple functions in order to avoid
+    /// needless allocator thrashing.
+    pub fn new() -> Context {
+        Context { func: Function::new() }
+    }
+}