Finish the work on try_apply

[Brezak]

Objective

Finish the try_apply implementation started in #6770 by @feyokorenhof.
Supersedes and closes #6770. Closes #6182

Solution

Add try_apply to Reflect and implement it in all the places that implement Reflect.

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Changelog

Added try_apply to Reflect.

[Brezak]

Current TODO:

• One last cleanup pass over the try_apply implementations.
• Figure out how to properly handle errors
  • Figure out which fields of the enum can be turned into static strings.
  • Maybe switch to a opaque struct with a kinds function.
• Write tests using try_apply (apply defaults to using try_apply but we should test if the proper errors are returned).

[Brezak]

The code is mostly finished. I'm just not confident about ApplyError. Strictly speaking its error messages are more correct than what the original panics would report. It's errors specify both types when there's a mismatch and will specify what kind of structure we're working with for all reflect types. On the other hand they more often fail to mention what field our operation failed at. My biggest concern is interaction with FromReflect in enum variant construction where we have to guess why exactly from_reflect failed.

[MrGVSV]

Looks good! Just a few comments/suggestions.

I also added suggestions to move some of the macro code to use fully-qualified syntax (hopefully with the bulk-apply for PR suggestions, that can take some of the work off your plate).

[soqb]

all this LGTM. just two little things before i approve.