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/module/src/module.rs

@@ -6,7 +6,7 @@
 // shared with `DataContext`?
 
 use cranelift_codegen::entity::{EntityRef, PrimaryMap};
-use cranelift_codegen::{binemit, ir, CodegenError, Context};
+use cranelift_codegen::{binemit, ir, isa, CodegenError, Context};
 use data_context::DataContext;
 use std::borrow::ToOwned;
 use std::collections::HashMap;
@@ -340,9 +340,10 @@ where
         self.names.get(name).cloned()
     }
 
-    /// Return then pointer type for the current target.
-    pub fn pointer_type(&self) -> ir::types::Type {
-        self.backend.isa().pointer_type()
+    /// Return the target information needed by frontends to produce Cranelift IR
+    /// for the current target.
+    pub fn target_config(&self) -> isa::TargetFrontendConfig {
+        self.backend.isa().frontend_config()
     }
 
     /// Create a new `Context` initialized for use with this `Module`.
@@ -351,7 +352,7 @@ where
     /// convention for the `TargetIsa`.
     pub fn make_context(&self) -> Context {
         let mut ctx = Context::new();
-        ctx.func.signature.call_conv = self.backend.isa().flags().call_conv();
+        ctx.func.signature.call_conv = self.backend.isa().default_call_conv();
         ctx
     }
 
@@ -361,14 +362,14 @@ where
     /// convention for the `TargetIsa`.
     pub fn clear_context(&self, ctx: &mut Context) {
         ctx.clear();
-        ctx.func.signature.call_conv = self.backend.isa().flags().call_conv();
+        ctx.func.signature.call_conv = self.backend.isa().default_call_conv();
     }
 
     /// Create a new empty `Signature` with the default calling convention for
     /// the `TargetIsa`, to which parameter and return types can be added for
     /// declaring a function to be called by this `Module`.
     pub fn make_signature(&self) -> ir::Signature {
-        ir::Signature::new(self.backend.isa().flags().call_conv())
+        ir::Signature::new(self.backend.isa().default_call_conv())
     }
 
     /// Clear the given `Signature` and reset for use with a new function.
@@ -376,7 +377,7 @@ where
     /// This ensures that the `Signature` is initialized with the default
     /// calling convention for the `TargetIsa`.
     pub fn clear_signature(&self, sig: &mut ir::Signature) {
-        sig.clear(self.backend.isa().flags().call_conv());
+        sig.clear(self.backend.isa().default_call_conv());
     }
 
     /// Declare a function in this module.