Clean up naming

[udoprog]

The following trait changes have been done, because I worked out a way to use a single trait independent of if the function is async or not. This is done by using marker trait implementation, the marker trait implements FunctionKind which allows for some introspection on the kind of function it is (only FunctionKind::is_async() for now).

• AsyncFunction (replaced by Function<A, K> where K: FunctionKind.
• InstFn and AsyncInstFn (replaced by InstanceFunction<A, K> where K: FunctionKind.

The following renames and deprecations have happened:

• Module::inst_fn and Module::async_inst_fn deprecated and replaced by Module::associated_function.
• Module::async_fn deprecated and replaced by Module::function.
• Module::field_fn deprecated and replaced by Module::field_function.
• Module::index_fn deprecated and replaced by Module::index_function.
• Function::function and Function::async_function deprecated and replaced by Function::new.