SDK: New BigInt types with conversion to decimal and integers

[sophialittlejohn]

Create extractions of BigInt that allows parsing to number and decimal and that uses BigInt under the hood (instead of BN.js).

Previously:

• CurrencyBalance
• TokenBalance (very similar to CurrencyBalance, check if we can improve naming to only use one or the other)
• Rate (27 decimals)
• Perquintill (mark as deprecated since it's only polkadot)
• Price

Tasks

• copy over logic from BN.ts but implement without BN.js library
• make classes only used in Polkadot context as deprecated
• when basic math is done on class it should return a new instance of the class to preserve types and methods (unlike currently when math is done on CurrencyBalance.add returns BN instead of CurrencyBalance).