This commit adds a hook to the `ModuleEnvironment` trait to learn when a
custom section in a wasm file is read. This hook can in theory be used
to parse and handle custom sections as they appear in the wasm file
without having to re-iterate over the wasm file after cranelift has
already parsed the wasm file.
The `translate_module` function is now less strict in that it doesn't
require sections to be in a particular order, but it's figured that the
wasm file is already validated elsewhere to verify the section order.
