Feature request: support user-defined macros written as SQL

[plaflamme]

Currently, user-defined macros need to be defined as python. An alternative SQL-based syntax would be more consistent with MODEL and AUDIT. That is, having a MACRO function to define user-defines macros in .sql files:

MACRO(
  name forall,
  defaults (
    condition NULL
  )
);
SELECT *
FROM @this_model
WHERE
  @AND(
    @REDUCE(@criteria, (l,r) -> l AND r),
    @condition,
  )

For supporting named arguments, perhaps a more general arguments could work, i.e.: you'd have to define each argument and its default value (and maybe its type for coercion)?:

MACRO(
  name accepted_range,
  arguments (
    min_v NULL,
    max_v NULL,
    inclusive TRUE,
    condition NULL,
  )
);
...