Query comprehension macro

[Mr-Byte]

The query comprehension macro needs to support the following:

• from clauses
• multiple from clauses
  • carry over context from multiple from clauses into other clauses
• where clauses
• let clauses
• sorting clauses
  • orderby
  • ascending/descending
  • sub-sorting clauses
• group by clauses
• join clauses
• into clauses
  • for selects
  • for group by
  • for join

The following extensions would be useful, but not required:

• where let clauses (combined filter + let clause) (dropped for the time being)
• do clauses (calls Iterator::inspect)
• alternative end clauses
  • max by
  • min by
  • first
  • single
  • skip/take
    • into clause
  • partition by
    • into clause
  • fold/aggregate
  • where all
  • where any
  • collect
  • foreach (experimental design idea)

[Mr-Byte]

The way where let clauses is currently implemented requires each clause to be implemented twice. I need to do some bench marking to compare adding the match with not adding the match to clauses that don't need to have it. If there's negligible performance difference, I'll probably just include the match in every clause regardless of need and drop the current implementation of only switching over to the match when a where let clause first appears.

[Mr-Byte]

In the interest of progress I think I'll cut the where-let clauses from initial phases. It's proving to be difficulty to be maintainable.