add TryFrom impls to convert from hir to encoder types (#424)

[zackangelo]

Took a shot at implementing these conversion impls. The issue mentioned that the approach required is dependent on some other decisions. Let me know if this aligns with the direction you'd like to take and I'll clean up and add some tests.

Fixes #424

[apollo-cla]

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[lrlna]

@zackangelo thank you for attempting to tackle this!

I think we need to scope out a few more use cases and a direction for #420 before committing to implementing #424. I am not super keen to add extra API surface area before then. I'll keep this PR open for now, and give you a shout if we end up needing a TryFrom for the encoder types.

[zackangelo]

Sounds great, please let me know if I can be of further help.

[lrlna]

@zackangelo i thought about it some more, and regardless to our decision on #420, this would be a useful addition. especially for testing :)