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.

[coeff-aij]

Originally I defined this struct to implement a parser for definitions of user-defined predicates. If the parser is implemented in the future, it might be fine to leave this definition as is.

[coord-e]

I’d like to avoid leaving unused structs because it makes the intent harder to understand later. Requirements might change by the time we write the parser for the definitions, so I don’t think there's a need to define them speculatively right now. (Either way, I think cargo clippy or something will probably flag it.)

[coeff-aij]

I reflected the comments in above review.