Fix trait codegen for list with uniqueitems trait

[joewyz]

Background

The current trait codegen is generating incorrect code for list with @uniqueItems. We use Set to represent this kind of list in generated Java code but some part of code still generate List related code.

For example, the following model:

$version: "2.0"

namespace example.traits

@trait
@uniqueItems
list MyListTrait {
    member: MyMemberList
}

@uniqueItems
list MyMemberList {
    member: String
}

will generate incorrect fromNode method because it still use Collection.toList()

public static MyListTrait fromNode(Node node) {
        Builder builder = builder();
        node.expectArrayNode()
            .getElements().stream()
            .map(n -> n.expectArrayNode()
                .getElements().stream()
                .map(n1 -> n1.expectStringNode().getValue())
                .collect(Collectors.toList()))
            .forEach(builder::addValues);
        return builder.build();
}

Similarly, if we have a list of unique items in a structure:

$version: "2.0"

namespace example.traits

@trait
structure MyStruct {
    member: MyMemberList
}

@uniqueItems
list MyMemberList {
    member: String
}

The generated Java code also failed because fromNode did not convert List to Set and getMemberOrEmpty returned emptyList

private final Set<String> member;
public static MyStructTrait fromNode(Node node) {
        Builder builder = builder();
        node.expectObjectNode()
            .expectArrayMember("member", n -> n.expectStringNode().getValue(), builder::member);

        return builder.build();
}

 public Set<String> getMemberOrEmpty() {
        return member == null ? Collections.emptyList() : member;
}

public static final class Builder extends AbstractTraitBuilder<MyStructTrait, Builder> {
        private final BuilderRef<Set<String>> member = BuilderRef.forOrderedSet();

        private Builder() {}

        public Builder member(Set<String> member) {
            clearMember();
            this.member.get().addAll(member);
            return this;
        }
}

This PR fixed the above problems.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sugmanue]

Let's make sure that we keep ordering of maps and sets of traits. That means to use a LinkedHashSet and LinkedHashMap for Set's and Map's respectively that would preserve the iteration order of how those were read or built.