Support loading individual FileDescriptorProtos

[andrewhickman]

Currently the entire set of file descriptors must be built into a FileDescriptorSet at once. It may be useful to add files to an existing set.

We can use Arc::make_mut to clone the FileDescriptor if anyone has references to it.

Currently the descriptor API also doesn't support inspecting the file structure at all, the closest we can get is looking at package_name.

[andrewhickman]

It may also be desirable to share a FileDescriptor between multiple other dependent FileDescriptor instances. For example if we store a static instance of the google.protobuf file descriptors to implement #10 then we could share that instance to avoid duplication.

[andrewhickman]

Copying DescriptorPool from the C++ API probably makes sense here. For example

struct DescriptorPool { ... }

impl DescriptorPool {
    ... existing FileDescriptorSet apis moved here ...

    fn new() -> DescriptorPool;

    fn add_file(&mut self, f: FileDescriptorProto);
    fn add_file_set(&mut self, f: FileDescriptorSet);
}