Add distinct, distinct_by built-ins for sequence

[clyoudu]

Add distinct, distinct_by built-ins for sequence which sub variable rules same as sort/sort_by.

Why?

• Data array with simple element type(string, number, boolean, date) often require deduplication，but there is no built-in for sequence deduplication. At present, it can only be achieved through cumbersome way such as the following:

<!-- sort first -->
<#assign checkImportList = checkImport?split("\n")?sort>
<!-- compare with prev item -->
<#list checkImportList as check>
    <#if check != "">
        <#if check?index <= 0 || check != checkImportList[check?index - 1]>
${check}
        </#if>
    </#if>
</#list>

and original order of elements discarded

• Sometimes data array with hash type require deduplication too.