Allow literals in statement templates (#287)

This PR is a continuation of the work done in #276 
- Fix PodType in MainPod (we were using `MockMain` instead of `Main`)
- Update anchored keys in statement template arguments to only support wildcards in the origin and literal keys as the key.
  - Update the pest grammar accordingly
  - Update the parser accordingly
- Rewrite the eth_dos example in a recursive manner so that we use one recursive pod for every distance increment of 1.
  - I've also used the podlang to define the eth_dos custom predicates.  Currently all predicates are in a single batch (previously `eth_friend` was in a different batch).  With #286 we could define `eth_friend` in a different batch again.
    - I was feeling a bit creative and used a format macro to pass `Value`s from rust to the podlang code.
  - The eth_dos is now written using literals.  This resolves https://github.com/0xPARC/pod2/issues/255
- Remove `StatementArg::WildcardValue` in favor of `StatementArg::Literal`.  The `WildcardValue` was just a way to have some kind of typing for values that would be used as arguments in custom predicates.  Now that we can have literals in any statement this value can be anything, so I just removed the `WildcardValue` and use `Literal` instead.  On the backend it was already the case that both cases were treated the same way (after all, `WildcardValue` and `Literal` were 4 fields in the backend).
  - Added a new type for Value: `PodId` so that we can use it for custom predicates that take a pod id to be used in a wildcard
- Add a mock vd_set that is empty for tests that don't use plonky2; this allows running those tests individually without paying for the expensive work of calculating the vd for various circuits.
- rename StatementTmplArg::WildcardValue to StatementTmplArg::Wildcard

src/lang/grammar.pest

@@ -54,8 +54,8 @@ statement_arg_list = { statement_arg ~ ("," ~ statement_arg)* }
 
 statement = { identifier ~ "(" ~ statement_arg_list? ~ ")" }
 
-// Anchored Key: (SELF | ?Var)["key_literal" | ?KeyVar]
-anchored_key = { ( self_keyword | wildcard ) ~ "[" ~ (wildcard | literal_string) ~ "]" }
+// Anchored Key: ?Var["key_literal"]
+anchored_key = { wildcard ~ "[" ~ literal_string ~ "]" }
 
 // Literal Values (ordered to avoid ambiguity, e.g., string before int)
 literal_value = {