Support for Google protocol buffers in combination with serde.

The crate is split up into several logical parts.

• The descriptor module provides an API for managing dynamically loaded protocol buffer schemata.
• The value module provides structs that can hold any raw protocol buffer decoded data (but the representation is heavily coupled with a schema).
• The de module can be used to deserialize binary encoded protocol buffer messages given some schema descriptors.

Serialization is not yet implemented in this version.