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/backend.rs

@@ -3,9 +3,9 @@
 use DataContext;
 use Linkage;
 use ModuleNamespace;
+use ModuleError;
 use cretonne_codegen::Context;
 use cretonne_codegen::isa::TargetIsa;
-use cretonne_codegen::result::CtonError;
 use cretonne_codegen::{binemit, ir};
 use std::marker;
 
@@ -57,19 +57,17 @@ where
         ctx: &Context,
         namespace: &ModuleNamespace<Self>,
         code_size: u32,
-    ) -> Result<Self::CompiledFunction, CtonError>;
+    ) -> Result<Self::CompiledFunction, ModuleError>;
 
     /// Define a zero-initialized data object of the given size.
     ///
     /// Data objects must be declared before being defined.
-    ///
-    /// TODO: Is CtonError the right error code here?
     fn define_data(
         &mut self,
         name: &str,
         data_ctx: &DataContext,
         namespace: &ModuleNamespace<Self>,
-    ) -> Result<Self::CompiledData, CtonError>;
+    ) -> Result<Self::CompiledData, ModuleError>;
 
     /// Write the address of `what` into the data for `data` at `offset`. `data` must refer to a
     /// defined data object.