Add example of how to use custom type formatting from Nim

tools/debug/customdebugtype.nim

@@ -0,0 +1,48 @@
+## This is a demo file containing an example of how to
+## create custom LLDB summaries and objects with synthetic
+## children. These are implemented in Nim and called from the Python
+## nimlldb.py module.
+##
+## For summaries, prefix your proc names with "lldbDebugSummary", use
+## the `{.exportc.}` pragma, and return a string. Also, any `$` proc
+## that is available will be used for a given type.
+##
+## For creating a synthetic object (LLDB will display the children), use
+## the prefix "lldbDebugSynthetic", use the `{.exportc.}` pragma, and
+## return any Nim object, array, or sequence. Returning a Nim object
+## will display the fields and values of the object as children.
+## Returning an array or sequence will display children with the index
+## and value.
+
+import intsets
+
+type
+  CustomType* = object of RootObj # RootObj is not necessary, but can be used
+    myField*: int
+
+  CustomSyntheticReturn* = object
+    differentField*: float
+
+
+proc lldbDebugSummaryCustomType*(ty: CustomType): string {.exportc.} =
+  ## Will display "CustomType(myField: 0)" as a summary
+  result = "CustomType" & $ty
+
+proc lldbDebugSyntheticCustomType*(ty: CustomType): CustomSyntheticReturn {.exportc.} =
+  ## Will display the fields of CustomSyntheticReturn as children of
+  ## CustomType
+  result = CustomSyntheticReturn(differentField: ty.myField.float)
+
+proc lldbDebugSummaryIntSet*(intset: IntSet): string {.exportc.} =
+  ## This will print the object in the LLDB summary just as Nim prints it
+  result = $intset
+
+proc lldbDebugSyntheticIntSet*(intset: IntSet): seq[int] {.exportc.} =
+  ## This will create a synthetic object to make it so that IntSet
+  ## will appear as a Nim object in the LLDB debugger window
+  ##
+  ## returning a seq here will display children like:
+  ##
+  result = newSeqOfCap[int](intset.len)
+  for val in intset:
+    result.add(val)