Reduce number of crates needed for Config usage

This commit is an attempt to reduce the number of crates necessary to
link to when using `wasmtime::Config` in "default mode" or with only one
or two tweaks. The change moves to a builder-style pattern for `Config`
to only require importing crates as necessary if you configure a
particular setting. This then also propagates that change to `Context`
as well by taking a `Config` instead of requiring that all arguments are
passed alone.

crates/api/tests/import_calling_export.rs

@@ -24,7 +24,7 @@ fn test_import_calling_export() {
         }
     }
 
-    let engine = HostRef::new(Engine::new(Config::default()));
+    let engine = HostRef::new(Engine::default());
     let store = HostRef::new(Store::new(&engine));
     let module = HostRef::new(
         Module::new(