API usability improvements

[tiffany352]

• Added helper functions load_definitions() and load_currency() on Context to deduplicate code every frontend had to write out
• Added ToSpans impl for Result<QueryReply, QueryError> to avoid a pointless looking match statement
• Added more examples to the API docs
• Fleshed out the API docs a bit more

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.57143% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 82.55%. Comparing base (df8b961) to head (89deeb2).

Files	Patch %	Lines
cli/src/config.rs	66.66%	5 Missing :warning:
rink-js/src/lib.rs	0.00%	3 Missing :warning:
core/src/output/reply.rs	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #184 +/- ## ========================================== + Coverage 81.97% 82.55% +0.58% ========================================== Files 42 42 Lines 7632 7636 +4 ========================================== + Hits 6256 6304 +48 + Misses 1376 1332 -44 ```

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