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/simplejit/tests/basic.rs

@@ -5,7 +5,7 @@ extern crate cranelift_module;
 extern crate cranelift_simplejit;
 
 use cranelift_codegen::ir::*;
-use cranelift_codegen::settings::*;
+use cranelift_codegen::isa::CallConv;
 use cranelift_codegen::Context;
 use cranelift_entity::EntityRef;
 use cranelift_frontend::*;