Add doc aliases

[Taaitaaiger]

Most methods in jlrs map to one or more functions in the Julia C API. In order to make jlrs more accessible to people who are already familiar with this API, I want to add doc aliases to let users find the appropriate method in jlrs by searching for the name used in the C API.

For example, the Value struct would be marked with #[doc(alias = "jl_value_t")], and Datatype::instantiate with #[doc(alias = "jl_new_structv")]