[FeatureRequest] Add convenience proc that fetches Many-To-X relationships of multiple entries within one SQL query
Short example:

Say you have a many-to-many relationship of users in groups. You have a list of users and want to have all their groups. Currently, you'd need to loop over that list of users and call the "getManyToMany" proc for every individual user, meaning one query to the database per user. 

That is inefficient. The alternative would be to write a normal `select` query yourself to fetch all necessary entries in one go, but that's very repetitive work if it occurs multiple times and I'd like to have a convenience proc.
Seeing as my sideproject has multiple of these issues, I'm pretty motivated to find a solution.

I'll likely provide a PR in the coming weeks/months once I have an idea on how I'd implement this feature in a way that fits into norm. 

My first draft is this (this is not functional code, particularly because of the last line):
```nim
import norm/[model, sqlite]
import std/[sugar, sequtils, tables, strformat, strutils]
proc selectOneToMany*[O: Model, M: Model](dbConn; oneEntries: seq[O], relatedEntries: var seq[M], foreignKeyFieldName: static string): Table[int64, seq[M]] =
  let initialIds: seq[int64] = oneEntries.map(entry => entry.id)
  let initialIdsStr: string = initialIds.map(id => id.int.intToStr()).join(",")
  let condition = fmt"{oneEntries.table()}.{foreignKeyFieldName} IN ({initialIdsStr})"
  connection.select(relatedEntries, condition)
  
  for initialId in initialIds:
    let id = initialId #potentially necessary to avoid compiler complaining about lent values used in anonymous procs
    result[initialId] = relatedEntries.filter(entry => entry.{foreignKeyFieldName} == id) #<-- I'm not quite sure yet how to do that one, likely I'll require a macro
```

However I end up solving the problem for `selectOneToMany`, I'll solve it similarly for `selectManyToMany`.

It's fairly certain that this will not be 1:1 how I'll implement this, this is just a first draft of an idea and not in any shape finalized. There's still a lot of refinement to be put into this solution.
Maybe I'll have the user pass in the table instead of the seq or something.