Update is_open flag to check for new stakers

[raychu86]

Motivation

This PR updates credits.aleo in two ways:

1. Change is_open check to prevent bonds from new stakers only. Existing stakers can still bond to the validator if this flag is set to false.

   • This helps with validator UX, especially if the validator only wants a specific set of delegators. (e.g. a special delegation program - ARC-38: Create Delegated Staking Standard Program with Commission ARCs#66)
   • This is safe because it does not prevent validators from unbonding their delegators with unbond_delegator_as_validator.

2. Remove 10 validator maximum in credits.aleo

   • This check is now moved to VM::speculate. Where bond_public calls that would cause the committee size to exceed MAX_COMMITTEE_SIZE will be rejected.
   • This unifies the interface for where the max committee size is set to just the MAX_COMMITTEE_SIZE variable.

Test Plan

Tests have been added to ensure the following:

1. New delegators can't bond to validators if the is_open flag is set to false.
2. Existing delegators can bond to validators if the is_open flag is set to false.
3. A validator can bond more stake to itself no matter what the is_open flag is set to.
4. bond_public transactions will be rejected if the would have caused the committee size to exceed the maximum.