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/simplejit/src/backend.rs

@@ -2,10 +2,9 @@
 
 use cretonne_codegen::binemit::{Addend, CodeOffset, Reloc, RelocSink, NullTrapSink};
 use cretonne_codegen::isa::TargetIsa;
-use cretonne_codegen::result::CtonError;
 use cretonne_codegen::{self, ir, settings};
 use cretonne_module::{Backend, DataContext, Linkage, ModuleNamespace, Writability,
-                      DataDescription, Init};
+                      DataDescription, Init, ModuleError};
 use cretonne_native;
 use std::ffi::CString;
 use std::ptr;
@@ -116,7 +115,7 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
         ctx: &cretonne_codegen::Context,
         _namespace: &ModuleNamespace<Self>,
         code_size: u32,
-    ) -> Result<Self::CompiledFunction, CtonError> {
+    ) -> Result<Self::CompiledFunction, ModuleError> {
         let size = code_size as usize;
         let ptr = self.code_memory.allocate(size).expect(
             "TODO: handle OOM etc.",
@@ -139,7 +138,7 @@ impl<'simple_jit_backend> Backend for SimpleJITBackend {
         _name: &str,
         data: &DataContext,
         _namespace: &ModuleNamespace<Self>,
-    ) -> Result<Self::CompiledData, CtonError> {
+    ) -> Result<Self::CompiledData, ModuleError> {
         let &DataDescription {
             writable,
             ref init,