Prefix all newly generated names with `c2rust_`

[Rua]

Whenever the transpiler generates a new name that is not in the original source code, prefix it with c2rust_. This makes it much clearer when reviewing/refactoring the generated code, what is original and what is a transpilation artefact. This would not apply to original names that were renamed due to a naming conflict, only to names that are created anew by the transpiler.