Improvements to Modules API (#307)

* test-no_std: use cargo +nightly

assume folks have rustup set to use stable by default

* cretonne-module, -faerie, -simplejit: use new ModuleError enum

CtonError is not really appropriate for use in the module system.
Instead, create a new enum ModuleError, which implements failure::Fail
(works with no_std). Translate existing panics and unimplemented
error cases to return ModuleErrors.

* cretonne-faerie: export FaerieProduct

* cretonne-module: expose FuncOrDataId, and Module::get_name to lookup

This is helpful for looking up a name that has already been declared.
Also, implement FuncOrDataId -> ExternalName conversion.

* cretonne-faerie: depend on faerie 0.3.0

which has bugfix for data relocations

* cretonne-module: change InvalidDefinition to InvalidImportDefinition

per dan's code review. plus another typo fix

* cretonne-faerie: add optional manifest of all traps from codegen

* cretonne-module: provide more context in panics

* cretonne-faerie: updates to docs

* cretonne-faerie: return an Err instead of debug_assert when isa not pic

lib/module/src/lib.rs

@@ -16,9 +16,12 @@
                 use_self,
                 ))]
 
+#[macro_use]
 extern crate cretonne_codegen;
 #[macro_use]
 extern crate cretonne_entity;
+#[macro_use]
+extern crate failure;
 
 mod backend;
 mod data_context;
@@ -26,4 +29,4 @@ mod module;
 
 pub use backend::Backend;
 pub use data_context::{DataContext, Writability, DataDescription, Init};
-pub use module::{DataId, FuncId, Linkage, Module, ModuleNamespace};
+pub use module::{DataId, FuncId, FuncOrDataId, Linkage, Module, ModuleNamespace, ModuleError};