Make process code generic

[alexdewar]

Description

This PR moves various bits of code out of process.rs and makes them more generic so that they can be used by other modules (e.g. it will be useful for the commodities code).

I've introduced a couple of changes to how things were implemented too:

1. I'm now using Rc<str> (reference counted strings) for ID values in many places. We end up using a lot of strings as the various process-related input files are read into HashMaps with IDs as strings and we want to avoid copying them as much as possible. An alternative is to use references with appropriate lifetimes but this is a bit more finnicky and doesn't always work (e.g. you can't have a key that refers to a field in the value of the same map).
2. I changed various functions for reading in and processing CSV data to work with iterators rather than directly with containers (e.g. Vec), which makes the code cleaner and more composable. Some of this has involved writing traits, which seemed odd to me at first, but I think the result is much nicer.

I also fixed #137 while I was at it.

Fixes #137.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 87.30159% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.85%. Comparing base (b7f3338) to head (7a128db).

Files	Patch %	Lines
src/input.rs	80.00%	6 Missing ⚠️
src/process.rs	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
- Coverage   80.36%   78.85%   -1.52%     
==========================================
  Files          10       10              
  Lines         163      175      +12     
==========================================
+ Hits          131      138       +7     
- Misses         32       37       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[alexdewar]

Actually, maybe hold off on reviewing this for now. I've thought of (what might be) a simpler way of doing things

[alexdewar]

It turned out to be a less simple idea than I thought 😛. I think this is good to go!

[alexdewar]

@tsmbland has signed off on this verbally, so I'm just going to go ahead and merge.