Frontend: simplify custom predicates interfaces (#83)

* add comments detailing logic, migrate middleware::custom::tests to frontend::custom

* simplify custom predicate's frontend interfaces, making it less verbose to define Statement Template arguments

The main idea is that when defining the arguments at a statement
template, it can be done from 3 different inputs:
i. `(&str, literal)`: this is to set a POD and a field, ie. `(POD, literal("field")`)
ii. `(&str, &str)`: this is to define a origin-key wildcard pair, ie. `(src_origin, src_dest)`
iii. `Value`: this is to define a literal value, ie. `0`

src/frontend/mod.rs

@@ -14,8 +14,10 @@ use crate::middleware::{
     PodSigner, SELF,
 };
 
+mod custom;
 mod operation;
 mod statement;
+pub use custom::*;
 pub use operation::*;
 pub use statement::*;