Unify update recovery logic under an update guard

[divagant-martian]

• Should mostly tackle #26 by unifying checks over spec keys that were previously done in the enr, in the handler, in the decoding code, and in a couple other functions in a single place: an Update::to_valid
• Unify the code for error recovery under a single update guard, this cleans up the enr, the insert, remove_insert, and decoding
• Adds an api that allows for an arbitrary number of generic updates (inserts and removals) that perform a single sequence number update and returns the previous values. Api looks like this:

  let updates = (
    Update::insert("some key", &<some Encodable>),
    Update::insert_raw("other key", <raw rlp>),
    Update::remove("stuff"),
  );
  let (prev0, prev1, prev2) = enr.update(updates, &key)?;

  this can be done for tuples of to size up to 7, single updates and vectors.

[divagant-martian]

The API I wanted to achieve needs features rust does not have yet. In an ideal world, I'd like to be able to do

let (prev_val1, prev_val2, prev_val3) = enr
    .update_guard()
    .insert(key1, value1)
    .insert(key2, value1)
    .remove(key3)
    .finish()?;

for arbitrary number of values

this intermediate state I achieved it not good enough to justify the complexity.

I'm thus closing