Handle non-bijective data-type conversions more robustly

[yeesian]

Given that we're using dictionaries for the code to convert between data types, there's no good way of asserting "default type comes last" for mappings that are not bijective.

Opening this ticket to

• Include it in the docstring for the convert macro.
• Add unit tests for them.
• Rewrite the implementation to enforce it by construction.

[yeesian]

Looks like it was discussed in e.g. https://discourse.julialang.org/t/initialising-a-dictionary-with-duplicate-keys/2290 in the past.

[mathieu17g]

Looks like it was discussed in e.g. https://discourse.julialang.org/t/initialising-a-dictionary-with-duplicate-keys/2290 in the past.

I will give it a try