[nim] Nim Generator Fixes

[winrid]

Normally I prefer more targeted PRs, but the Nim generator needed a lot of fixes to be usable for me, so here they are :)

1. Fixes trailing underscores in identifiers (I guess not allowed w/ Nim)
2. Fixes top-level enum generation (were empty objects instead of enums)
3. Fixes enum serialization for numeric values (only worked for strings previously)
4. Fixes import/filename consistency
5. Fixes object keyword conflicts (was using reserved keywords as types)
6. Adds helper types for AnyType and Object
7. Adds support for HTTP DELETE with request body (existing http lib doesn't seem to support this w/ same syntax as other method types)
8. Fixes record type references - underscored names in generic containers
9. support oneOf/anyOf via nim object variants
10. optional field support
11. Don't include empty optional parameters in URLs
12. integer enums support

@wing328

[wing328]

@winrid thanks for the PR

cc @hokamoto (author of the Nim client generator)

[hokamoto]

Thank you so much for your fix @winrid !

[wing328]

I'll run some tests later this week

If no issue found, I'll merge it.

[winrid]

added:

• support oneOf/anyOf via nim object variants
• optional field support
• Don't include empty optional parameters in URLs

[winrid]

I think there is a bug with fields like _id - fixing

[winrid]

Okay, now it's ready for review :) That got kind of wild to support nim's type system, but it works, and cleans up the generated models some. Enums now also support integer ordinals.