[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

@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.