Support user-defined predicate call in annotation

[coeff-aij]

This extention allows users to call arbitrary user-defined predicates (e.g., is_double(x, result)) within annotations.
Currently, a dedicated syntax for defining user-defined predicates is not yet supported. Therefore, users must define them using the #![thrust::raw_define()] (introduced in #21 ) attribute for now.

• Usage
  The predicate is_double() defined using #![thrust::raw_define()] can be called in the annotations such as #[thrust::ensures()]:

#![feature(custom_inner_attributes)]
#![thrust::raw_define("(define-fun is_double ((x Int) (doubled_x Int)) Bool
    (=
        (* x 2)
        doubled_x
    )
)")]

#[thrust::requires(true)]
#[thrust::ensures(is_double(x, result))]
fn double(x: i64) -> i64 {
    x + x
}

[coord-e]

This struct seems to be unused. I think it's more natural to rename UserDefinedPredSymbol to UserDefinedPred and remove this one.

[coord-e]

It's actually better to have different structs for different purposes; that makes them more resilient to change, since each might be updated for different reasons and in different ways in the future.

[coord-e]

(nit) using p.to_string().into() would be better because we don't have to access the implementation details (like .inner).

[coord-e]

Could you rename parse_datatype_ctor_args to parse_arg_terms or something similar, because it's no longer used solely for datatype constructor arguments?