Rust: Add support for lists (and sets) of enum values

[gferon]

For a field described like:

matrixAttributes:
  type: array
  uniqueItems: true
  items:
    type: string
    enum: ["travelTimes", "distances"]
  default: ["travelTimes"]

the generated field in Rust would not be correct and was:

#[serde(rename = "matrixAttributes", skip_serializing_if = "Option::is_none")]
pub matrix_attributes: Option<MatrixAttributes>,

with this patch, it is now correctly wrapped in a collection, depending on whether uniqueItems is set or not:

For a list:

#[serde(rename = "matrixAttributes", skip_serializing_if = "Option::is_none")]
pub matrix_attributes: Option<Vec<MatrixAttributes>>,

For a set:

#[serde(rename = "matrixAttributes", skip_serializing_if = "Option::is_none")]
pub matrix_attributes: Option<std::collections::HashSet<MatrixAttributes>>,

cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05) for review

[wing328]

  type: array
  uniqueItems: true

@gferon thanks for the PR. Are we on the same page that this is a set instead of array?

I've not checked if we have correctly mapped this to a proper set type in Rust in the generator.

[gferon]

@wing328 great catch! you're absolutely right, I've added a test for {{#uniqueItems}} now to support both sets and lists of enum values.

[wing328]

Tested locally and the result is good.

[wing328]

@gferon thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423