Rework stack-frame handling in the VM

Instead of having the stack frames as `ref` objects that are chained
together via their `next` field, store them in a `seq` in `TCtx`. This
makes it easier to reason about them, and also connects the frames
to their owning context.

While the frame list should ideally be first-in last-out (stack), this
isn't currently possible due to how exception handling is implemented.

In addition, the stack trace now always includes the entry function
and also the number of skipped frames. A nil access error when
no entry function exists (happens when a statement is evaluated)
is also fixed.

compiler/ast/reports.nim

@@ -251,6 +251,7 @@ type
         currentExceptionA*, currentExceptionB*: PNode
         traceReason*: ReportKind
         stacktrace*: seq[tuple[sym: PSym, location: TLineInfo]]
+        skipped*: int
 
       of rsemReportCountMismatch,
          rsemWrongNumberOfVariables:

compiler/front/cli_reporter.nim

@@ -527,13 +527,19 @@ proc reportBody*(conf: ConfigRef, r: SemReport): string =
 
     of rsemVmStackTrace:
       result = "stack trace: (most recent call last)\n"
-      for idx, (sym, loc) in r.stacktrace:
+      for idx in countdown(r.stacktrace.high, 0):
+        let (sym, loc) = r.stacktrace[idx]
         result.add(
           conf.toStr(loc),
           " ",
-          sym.name.s,
-          if idx == r.stacktrace.high: "" else: "\n"
+          if sym != nil: sym.name.s else: "???"
         )
+        if r.skipped > 0 and idx == r.stacktrace.high:
+          # The entry point is always the last element in the list
+          result.add("\nSkipped ", r.skipped, " entries, calls that led up to printing")
+
+        if idx > 0: 
+          result.add("\n")
 
     of rsemVmUnhandledException:
       result.addf(