convertToDSNPUserURI and convertToDSNPUserId replacing BigNumber methods

[wilwade]

Problem

We don't want to have ethers BigNumber used outside of the sdk, so better methods to cast these to the correct formats.

Also prep for switching DSNP User Id to bigint

Solution

Replaced convert in and out of BigNumber methods with just convertToDSNPUserURI and convertToDSNPUserId

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?

Change summary:

• Removed convertDSNPUserURIToDSNPUserId
• Removed convertBigNumberToDSNPUserId
• Removed convertDSNPUserIdOrURIToBigNumber
• Removed convertBigNumberToDSNPUserURI
• Added convertToDSNPUserURI
• Added convertToDSNPUserId