migrate from anyhow to thiserror (#197)

* migrate from anyhow to thiserror (#190). pending polish error msgs

* Add backtrace and compartmentalize errors

- Include backtraces in the errors we generate.  To get this we can't
  just return a literal enum, because the backtrace requires a call.
- Related to the previous point: add methods to create errors so
  we can include the backtrace conveniently without changing too much
  the syntax.  So instead of `Err(Error::KeyNotFound(key))` (literal
  enum) it will be `Err(Error::key_not_found(key))` (method call)
- Each error should be local to its scope, and each scope should
  only return its own error.
  - The merkle tree should return `TreeError` and not Error
  - The middleware should return `MiddlewareError` and not Error
- With a global Error we can't easily include backend/frontend types in
  the error fields, so declare a `BackendError` and a `FrontendError`
  and follow the pattern from the previous point
- The Pod traits should be able to return backend errors and will be
  used in the frontend; for that we change them to use trait object
  Error: `dyn std::error::Error`

* fix error

* apply suggestions from @arnaucube

* rename XError and XResult to Error and Result

* reorg signature

* make frontend custom error more ergonomic

* remove unnecessary feature

---------

Co-authored-by: Eduard S. <eduardsanou@posteo.net>

src/backends/plonky2/mainpod/statement.rs

@@ -1,10 +1,10 @@
 use std::fmt;
 
-use anyhow::{anyhow, Result};
 use serde::{Deserialize, Serialize};
 
-use crate::middleware::{
-    self, NativePredicate, Params, Predicate, StatementArg, ToFields, WildcardValue,
+use crate::{
+    backends::plonky2::error::{Error, Result},
+    middleware::{self, NativePredicate, Params, Predicate, StatementArg, ToFields, WildcardValue},
 };
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -36,7 +36,7 @@ impl ToFields for Statement {
 }
 
 impl TryFrom<Statement> for middleware::Statement {
-    type Error = anyhow::Error;
+    type Error = Error;
     fn try_from(s: Statement) -> Result<Self> {
         type S = middleware::Statement;
         type NP = NativePredicate;
@@ -78,7 +78,10 @@ impl TryFrom<Statement> for middleware::Statement {
                 (NP::MaxOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3))), 3) => {
                     S::MaxOf(ak1, ak2, ak3)
                 }
-                _ => Err(anyhow!("Ill-formed statement expression {:?}", s))?,
+                _ => Err(Error::custom(format!(
+                    "Ill-formed statement expression {:?}",
+                    s
+                )))?,
             },
             Predicate::Custom(cpr) => {
                 let vs: Vec<WildcardValue> = proper_args