add fn macro alternative for json interoperability

The default `f{}` macro does not work if `json` is imported :/ See
issue #28 for reference.

src/ggplotnim/formula.nim

@@ -1282,10 +1282,15 @@ proc buildFormula(n: NimNode): NimNode =
  else:
  raise newException(Exception, "Not implemented! " & $n.kind)
 
-macro `{}`*(x, y: untyped): untyped =
- if x.repr == "f":
+macro `{}`*(x: untyped{ident}, y: untyped): untyped =
+ if x.strVal == "f":
  result = buildFormula(y)
 
+macro `fn`*(x: untyped): untyped =
+ let arg = if x.kind == nnkStmtList: x[0] else: x
+ expectKind arg, nnkCurly
+ result = buildFormula(arg[0])
+
 proc unique*(v: PersistentVector[Value]): seq[Value] =
  ## returns a seq of all unique values in `v`
  result = v.vToSeq.deduplicate

tests/test_issue28.nim

@@ -0,0 +1,19 @@
+import ggplotnim
+import json
+
+template accept(x) =
+ static: assert(compiles(x))
+
+template reject(x) =
+ static: assert(not compiles(x))
+
+accept:
+ let f = fn {"Channel" == "Ch 0"}
+accept:
+ let f2 = fn({"Channel" == "Ch 0"})
+accept:
+ let f3 = fn:
+ {"Channel" == "Ch 0"}
+
+reject:
+ let f = f{"Channel" == "Ch 0"}