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/data_context.rs

@@ -94,7 +94,7 @@ impl DataContext {
         self.description.init = Init::Zeros { size };
     }
 
-    /// Define a zero-initialized object with the given size.
+    /// Define an object initialized with the given contents.
     ///
     /// TODO: Can we avoid a Box here?
     pub fn define(&mut self, contents: Box<[u8]>, writable: Writability) {