Support Nim 2.x, compatibility with waterpark.

- Nim 2.x has moved the DB connectors outside the standard library to
  the `db_connector` package.
- Refactor the pooling implementation and macro expectations to use the
  `withConnection` name instead of `withConn`. This change allows a
  caller to use a [waterpark](https://github.com/guzba/waterpark) pool
  instance instead of the builtin pool instance. Waterpark provides
  better support for multi-threaded environments. The builtin pooling
  mechanism may be deprecated in favor of waterpark in the future.
- Add the `getModelIfItExists` generated proc to the list of standard
  procs we generate. This is a flavour of `getModel` that returns an
  `Option` instead of raising an exception when there is no model for
  the given id.
- Change `PaginationParams#orderBy` to accept a `seq[string]` to allow
  for sorting on multiple fields.

README.rst

@@ -57,7 +57,7 @@ Models may be defined as:
 
 .. code-block:: Nim
    # models.nim
-   import std/options, std/times
+   import std/[options, times]
    import uuids
 
    type
@@ -82,6 +82,8 @@ Using Fiber ORM we can generate a data access layer with:
 
 .. code-block:: Nim
    # db.nim
+   import std/[options]
+   import db_connectors/db_postgres
    import fiber_orm
    import ./models.nim
 
@@ -102,6 +104,7 @@ This will generate the following procedures:
 
 .. code-block:: Nim
    proc getTodoItem*(db: TodoDB, id: UUID): TodoItem;
+   proc getTodoItemIfItExists*(db: TodoDB, id: UUID): Option[TodoItem];
    proc getAllTodoItems*(db: TodoDB): seq[TodoItem];
    proc createTodoItem*(db: TodoDB, rec: TodoItem): TodoItem;
    proc updateTodoItem*(db: TodoDB, rec: TodoItem): bool;
@@ -112,6 +115,7 @@ This will generate the following procedures:
      values: varargs[string, dbFormat]): seq[TodoItem];
 
    proc getTimeEntry*(db: TodoDB, id: UUID): TimeEntry;
+   proc getTimeEntryIfItExists*(db: TodoDB, id: UUID): Option[TimeEntry];
    proc getAllTimeEntries*(db: TodoDB): seq[TimeEntry];
    proc createTimeEntry*(db: TodoDB, rec: TimeEntry): TimeEntry;
    proc updateTimeEntry*(db: TodoDB, rec: TimeEntry): bool;