Add Model trait to map logical models with Rust types

[coord-e]

for #57

Introduce Model trait and Model::Ty associated type to map Rust types to logical representation in Thrust for:

1. Uniformly translate Rust types to logical representations in annotation-only Rust functions Pre/post-conditions annotations using Rust expressions #57
2. Manage logical representation of std-types like Vec in std.rs (follow-up of Support some of std::vec::Vec methods #50)