Support for data joins and positional variables

[andrew-coleman]

This PR introduces two new operators that can be used within a path expression:

• @$var is used on a map step (.) to bind a variable to the current context item.
• #$var is used to bind a variable to the current context position (in the sequence).

The former can be used to perform data cross referencing and joins - resolves #333.
The latter resolves #187
The documentation has been updated with details of these operators, plus a section on the explaining path processing model.

[coveralls]

Coverage remained the same at 100.0% when pulling 828ae26 on join into db7cac7 on master.

[mattbaileyuk]

A couple of really minor points, but happy with all the rest, so will approve this because those are small things that could be fixed later anyway.

[jugaadi]

Can't we reduce it to just @var?

library.loans@l.books@b[$l.isbn=$b.isbn].{
  'title': $b.title,
  'customer': $l.customer
}

Same with #var

library.books#i['Kernighan' in authors].{
  'title': title,
  'index': $i
}

Alternative : An underscore to represent context computation. It can be extended just like functions.
Syntax : _<keyword specific symbol><var>, _<keywords>$<var>, or _<keyword> for current context.
Context Bindings :

• Index - _@i or _index$i or $_index
• Instance - _#o or _instance$o or $_instance
• Type - _%t or _type$t or $_type
• Parent - _$ or _parent$p or $_parent

Example :

library.books_#i['Kernighan' in authors].{
  'title': title,
  'index': $i
}

library.loans_@l.books_@b[$l.isbn=$b.isbn].{
  'sno': $_index
  'title': $b.title,
  'customer': $l.customer
}

[andrew-coleman]

Thanks for the feedback @jugaadi. In one of my early prototypes, I did experiment with dropping the '$'. However, I felt that the dollar made the variable token consistent across the language. It is also used when binding variables using the := operator, even though it could be argued that that is not necessary either.