changed the loans map key to function properly

[sternbergm]

As a tester, I was trying to build the lagoon app in order to interact with the sBTC contract. I encountered an issue when attempting to deploy the contract, which led me to an error in the way the loans map was being initialized.

solves issue #106 which I opened recently.

For details refer to issue #123

- API reference/documentation update

## Testing information

no testing required. 

## Checklist
- [na] Code is commented where needed
- [na] Unit test coverage for new or modified code paths
- [na] `npm run test` passes
- [na] Changelog is updated
- [na] Tag 1 of @person1 or @person2

[CLAassistant]

All committers have signed the CLA.

[friedger]

A better (cheaper) solution would be to replace the tuple with single property owner with the principal in the map-get and map-set and map-delete

[sternbergm]

ok. let me try that and push another commit

[sternbergm]

@friedger what about now? looks good?

[sternbergm]

@kenrogers could you help me get this to the right person to merge? thanks!