API: Add a representation for constant values (for semantic nodes)

[xFrednet]

The API distinguishes between syntactic and semantic types. Syntactic types, like arrays, can have constant expressions as their index. In Marker these are called ConstExpr and are equivalent to rustc's AnonConst.

Semantic types and generic arguments can also contain constant values, which are the semantic equivalent to ConstExpr. Rustc uses rustc_middle::ty::consts::Const with a lot of fun nested variants. In the API I've now added a ConstValue struct as a placeholder, to add a proper representation later.