[NES-218] update smart contracts to use interfaces

[eyqs]

What's new in this PR?

For example, instead of typing currencyToken as ERC20, type it as IERC20 instead within the contract itself and within all signatures and interfaces:

• Cast between IComponentToken and IERC20 for CurrencyToken in Nest contracts
• Remove unused ADMIN_ROLE in $P
• Remove unused import when deploying $pUSD
• Change some functions from public to external
• Change i++ to ++i
• Add named returns for impl and hasCode
• Update comments on isContract to be more accurate

Why?

What problem does this solve?
Why is this important?
What's the context?

[linear]

NES-218

[Purva-Chaudhari]

LGTM