refactor: Use near_sdk::store instead of the legacy near_sdk::collections

[njelich]

The example implementation used a legacy collection, instead of the newer ones located in near-sdk::store. I updated the implementation, and cleaned up feature flags.

Should I add the wee_alloc feature as well?

[frol]

I believe wee_alloc should only be disabled if you want to manually specify some other allocator. This is definitely not what we want to do in the simple starter app.