better nativestacktrace support; refs #15284; backport [1.2] (#15384)

* nimStackTraceOverride: enable stack traces in exceptions

This is a two-step stack trace collection scheme, because re-raised
exceptions will collect multiple stack traces but use them rarely, when
printing info about an uncaught exception, so it makes sense to only do
the cheap stack unwinding all the time and the relatively expensive
debugging information collection on-demand.

`asyncfutures` implements its own `$` proc for printing
`seq[StackTraceEntry]`, so we have to add the debugging info there, just
like we do for the private `$` proc in `system/excpt`.

* cleaned up PR #15284

Co-authored-by: Ștefan Talpalaru <stefantalpalaru@yahoo.com>

lib/system.nim

@@ -1383,6 +1383,12 @@ type # these work for most platforms:
   culonglong* {.importc: "unsigned long long", nodecl.} = uint64
     ## This is the same as the type ``unsigned long long`` in *C*.
 
+  # There is a disparity on macOS where Nim's `uint` is `unsigned long long` and
+  # `uintptr_t` is `unsigned long`. Even though both data types are the same
+  # size (64 bits), clang++ refuses to do automatic conversion between them.
+  cuintptr_t* {.importc: "uintptr_t", nodecl.} = uint
+    ## This is the same as the type ``uintptr_t`` in *C*.
+
   cstringArray* {.importc: "char**", nodecl.} = ptr UncheckedArray[cstring]
     ## This is binary compatible to the type ``char**`` in *C*. The array's
     ## high value is large enough to disable bounds checking in practice.