Allow entries as args in custom statements (#498)

- Introduce a new operation ReplaceValueWithEntry that allows taking any statement and replacing literal arguments with entries given a matching Contains statement.
- Allow entries as args in custom statements
- Circuit optimization: For the public statements slots in the circuit we only support None and Copy which take at most 1 argument; but we were still doing max_statement_args random accesses per slot; so I reduced that to just 1 random access to a previous statement.

src/lang/error.rs

@@ -135,12 +135,6 @@ pub enum ValidationError {
         span: Option<Span>,
     },
 
-    #[error("Invalid argument type for {predicate}: anchored keys not allowed")]
-    InvalidArgumentType {
-        predicate: String,
-        span: Option<Span>,
-    },
-
     #[error("Duplicate wildcard in predicate arguments: {name}")]
     DuplicateWildcard { name: String, span: Option<Span> },