[bug] `overloading = true` causes clashes when `enum` field and `const` have the same name

[heysokam]

# Errors
proc rename (
    name        : string;
    kind        : futhark.SymbolKind;
    partOf      : string;
    overloading : var bool;
  ) :string=
  overloading = true

importc:
  renameCallback rename
  outputPath currentSourcePath.parentDir/"result.nim"
  "vulkan/vulkan_core.h"

futhark.nim(420, 11) Error: redefinition of 'VK_STRUCTURE_TYPE_SURFACE_CAPABILITIES2_EXT'; previous declaration here: futhark.nim(420, 11)

# Works fine
importc:
  outputPath currentSourcePath.parentDir/"result_raw.nim"
  "vulkan/vulkan_core.h"

# Hints at the problematic name being renamed:
futhark.nim(249, 5) Hint: Renaming "VK_STRUCTURE_TYPE_SURFACE_CAPABILITIES2_EXT" to "VK_STRUCTURE_TYPE_SURFACE_CAPABILITIES2_EXT_enumval" [User]

Real usage example can be found at:
https://github.com/heysokam/vulkan.nim/tree/master/gen

[heysokam]

While working on https://github.com/heysokam/wgpu I found that, since enums in futhark are not marked pure, there is a clash between WGPUInstance (renamed to Instance from struct_WGPUInstance to enum_Name.Instance, because non-pure enums can cause namespace collisions.

This might be the same reason for the nameclash I reported happening with Vulkan.