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]

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 ⚠️
rink-js/src/lib.rs	0.00%	3 Missing ⚠️
core/src/output/reply.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            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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