[WIP] Module API (#294)

* Initial skeleton.

* Add basic faerie support.

This adds enough functionality to enable simple .o file writing through
faerie. This included adding the functionality to Module to support
RelocSink implementations.

* Add basic SimpleJIT support.

This adds enough functionality to enable a simple program to be jitted
and executed.

* Make declare_func_in_func take a Function instead of a Context.

It only needs the Function, and sometimes it's useful to call it from
places that don't have a full Context.

* Temporarily disable local and exported global variables in the Faerie backend.

Faerie assumes these variables use pc-relative offset instructions, and
Cretonne is not yet emitting those instructions.

* FaerieBackend depends on PIC.

Faerie itself only supports PIC objects for now, so add an assert to
Cretonne to check that it's using a PIC target flag.

* SimpleJIT support for data objects.

* Preliminary faerie support for data objects.

* Support for data objects in faerie using the new colocated flag.

* Unit tests for DataContext and friends.

* Add a Module::consume() function.

This consumes the Module and returns the contained Backend, so that
users can call Backend-specific functions with it. For example, the
Faerie backend has functions to write an object file.

* Update the new crates to version 0.4.4.

* Make FaerieBackend own its TargetIsa.

This simplifies its interface, as it eliminates a lifetime parameter.
While we may eventually want to look into allowing multiple clients to
share a TargetIsa, it isn't worth the complexity for FaerieBackend
right now.

* Don't try to protect faerie from multiple declarations.

Let faerie decide for itself whether it wants to consider two
declarations to be compatible.

* Use debug_assert_eq rather than debug_assert with ==.

* Fix FaerieRelocSink's reloc_external to handle data object names.

* Relax the asserts in get_function_definition and get_data_definition.

These functions don't require definable symbols, but they do require
that definable symbols be defined. This is needed for the simplejit
backend.

* Add a function to the faerie backend to retrieve the artifact name.

* Sync up with cretonne changes.

lib/module/src/lib.rs

@@ -0,0 +1,15 @@
+//! Top-level lib.rs for `cretonne_module`.
+
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+
+extern crate cretonne_codegen;
+#[macro_use]
+extern crate cretonne_entity;
+
+mod backend;
+mod data_context;
+mod module;
+
+pub use backend::Backend;
+pub use data_context::{DataContext, Writability, DataDescription, Init};
+pub use module::{DataId, FuncId, Linkage, Module, ModuleNamespace};