Add pragma accessors

This library requires a forked macros module, so export accessors
that would prevent a downstream library from having the same
problems with std/macros.

src/preserves.nim

@@ -635,6 +635,18 @@ template preservesRecord*(label: string) {.pragma.}
   ## Serialize this object or tuple as a record.
   ## See ``toPreserve``.
 
+proc hasPreservesRecordPragma*(T: typedesc): bool =
+  ## Test if a type has a `{.preservesRecord: "…".}` pragma attached.
+  hasCustomPragma(T, preservesRecord)
+
+proc recordLabel*(T: typedesc): string =
+  ## Get the record label set by a pragma on a type.
+  runnableExamples:
+    type Foo {.preservesRecord: "bar".} = object
+      n: int
+    assert recordLabel(Foo) == "bar"
+  T.getCustomPragmaVal(preservesRecord)
+
 template preservesTuple*() {.pragma.}
   ## Serialize this object or tuple as a tuple.
   ## See ``toPreserve``.
@@ -643,10 +655,18 @@ template preservesTupleTail*() {.pragma.}
   ## Serialize this object field to the end of its containing tuple.
   ## See ``toPreserve``.
 
+proc hasPreservesTuplePragma*(T: typedesc): bool =
+  ## Test if a type has a `preservesTuple` pragma attached.
+  hasCustomPragma(T, preservesTuple)
+
 template preservesDictionary*() {.pragma.}
   ## Serialize this object or tuple as a dictionary.
   ## See ``toPreserve``.
 
+proc hasPreservesDictionaryPragma*(T: typedesc): bool =
+  ## Test if a type has a `preservesDictionary` pragma attached.
+  hasCustomPragma(T, preservesDictionary)
+
 template preservesOr*() {.pragma.}
   ## Serialize this object as an ``or`` alternative.
   ## See ``toPreserve``.