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/lib.rs

@@ -2,28 +2,30 @@
 
 #![deny(missing_docs)]
 
+pub use context::Context;
+pub use legalizer::legalize_function;
 pub use verifier::verify_function;
 pub use write::write_function;
-pub use legalizer::legalize_function;
 
 /// Version number of the cretonne crate.
 pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
-pub mod ir;
-pub mod isa;
 pub mod cfg;
 pub mod dominator_tree;
-pub mod entity_map;
 pub mod entity_list;
-pub mod sparse_map;
-pub mod settings;
-pub mod verifier;
+pub mod entity_map;
+pub mod ir;
+pub mod isa;
 pub mod regalloc;
+pub mod settings;
+pub mod sparse_map;
+pub mod verifier;
 
-mod write;
 mod constant_hash;
-mod predicates;
+mod context;
 mod legalizer;
-mod ref_slice;
-mod partition_slice;
 mod packed_option;
+mod partition_slice;
+mod predicates;
+mod ref_slice;
+mod write;