Implement ShallowInit gotoc codegen

[celinval]

Requested feature: The latest revision of rustc includes a new rvalue type named ShallowInit. We need to be able to codegen this new value type in order to be able to merge changes from the upstream repository.
Use case: N/A
Link to relevant documentation (Rust reference, Nomicon, RFC): More details on the semantics can be found here: rust-lang/compiler-team#460
Is this a breaking change?

Test case:

smack/vector/vec_resize.rs

This blocks the merge for week 38: #501