Making type IDs accessible from Rust code

These patches should allow the use of Capn'Proto type ids from Rust code.

For a Capn'Proto struct, it declares a `const TYPE_ID: u64` in the module defining the struct, and the `Reader` and `Builder` both implement the `HasTypeId` trait defining a `type_id(_ : Option<Self>)` function (no better way to call it before UFCS).

For an enum, it only makes the enum implement the `HasTypeId` trait.

For an interface, it declare a `const INTERFACE_ID: u64` in the module defining the interface, and the Client implements the `HasTypeId` trait

This pull request requires the HasTypeId trait in the runtime library.
Well I haven't done extensive testing, but the tests and benchmarks compile so that probably works.
However, I am worried about a possible performance impact: I hope that the dispatching of `::capnp::traits::HasStructSize::struct_size(None::<MyStruct>)` is static and not virtual (I don't know enough about Rust intrinsics to answer that)