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

cranelift/test-no_std.sh

@@ -21,10 +21,10 @@ do
     cd "lib/$LIB"
 
     # Test with just "core" enabled.
-    cargo test --no-default-features --features core
+    cargo +nightly test --no-default-features --features core
 
     # Test with "core" and "std" enabled at the same time.
-    cargo test --features core
+    cargo +nightly test --features core
 
     cd "$topdir"
 done