Implement JsonSchemaAs for EnumMap

[swlynch99]

It's been a little busy but I'm back with yet another PR :)

This is the first one that really involves a custom deserialize implementation. EnumMap involves transforming a list of enums to a map and to generate its schema we end up needing to do the same to the schema.

Basically, we end up transforming

{
  "type": "object",
  "oneOf": [
    { "properties": { "A": "..." } },
    { "properties": { "B": "..." } }
  ]
}       

into

{
  "type": "object",
  "properties": {
    "A": "...",
    "B": "..."
  }
}

I have also included a number of tests that I think should cover most of the different cases.

Notes

• If the schema for the inner type doesn't match what would be expected for a rust enum then we'll end up generating an invalid overall schema. I don't think this is an issue since it doesn't appear that EnumMap itself works in this case?

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (1be8fa3) 65.41% compared to head (2a65f20) 65.64%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
serde_with/src/schemars_0_8.rs	80.95%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
+ Coverage   65.41%   65.64%   +0.22%     
==========================================
  Files          38       38              
  Lines        2322     2343      +21     
==========================================
+ Hits         1519     1538      +19     
- Misses        803      805       +2     

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[jonasbb]

Hey, thank you for that. The transformation looks good. I think it is fine to not handle schema mismatches for the inner enum. At best, you could detect that something is going on and raise an error. The json_schema function is infallible, so it is not possible to nicely pass an error. Panicing would be a worse choice. EnumMap can only handle default style enums. I don't think there is much error handling in that case either, since nothing can be done really.