Add 'join' and 'anchored' RecordPath functions

I've come across two functions that would make flow design much simpler in RecordPath.

The first one, 'join' would be similar to the 'concat' method but provides a delimiter between each element instead of just smashing the values together.

The other provides the ability to anchor the context node while evaluating a RecordPath. For example, given the following record:

{
    "id": "1234",
    "elements": [{
        "name": "book",
        "color": "red"
    }, {
        "name": "computer",
        "color": "black"
    }]
} 

We should be able to use:

anchored(/elements, concat(/name, ': ', /color)) 

In order to obtain an array of 2 elements:

book: red 

and

computer: black