Introduce a TargetFrontendConfig type. (#570)

* Introduce a `TargetFrontendConfig` type.

`TargetFrontendConfig` is information specific to the target which is
provided to frontends to allow them to produce Cranelift IR for the
target. Currently this includes the pointer size and the default calling
convention.

The default calling convention is now inferred from the target, rather
than being a setting. cranelift-native is now just a provider of target
information, rather than also being a provider of settings, which gives
it a clearer role.

And instead of having cranelift-frontend routines require the whole
`TargetIsa`, just require the `TargetFrontendConfig`, and add a way to
get the `TargetFrontendConfig` from a `Module`.

Fixes #529.
Fixes #555.

lib/wasm/src/sections_translator.rs

@@ -35,7 +35,7 @@ pub fn parse_function_signatures(
                 ref params,
                 ref returns,
             }) => {
-                let mut sig = Signature::new(environ.flags().call_conv());
+                let mut sig = Signature::new(environ.target_config().default_call_conv);
                 sig.params.extend(params.iter().map(|ty| {
                     let cret_arg: ir::Type = type_to_type(*ty)
                         .expect("only numeric types are supported in function signatures");