Add num_children method to some gdb pretty-printers

gdb doesn't have a way to know when an object hasn't yet been
initialized, and in this case, if a pretty-printer returns an absurd
number of children, this can result in apparent hangs in some modes.
This came up specifically with DAP, see this bug report:

    https://sourceware.org/bugzilla/show_bug.cgi?id=33594

This patch (mostly) addresses this potential issue in the Rust
pretty-printers, by adding 'num_children' methods.  In particular a
method like this is added when the number of children is variable and
also relatively easy to compute.  (I.e., I didn't attempt the btree
printers.)

Supplying num_children is good for DAP regardless of the
initialization problem, because DAP requires a count of child objects
and this is more efficient than enumerating the children, which is
gdb's fallback approach.

Author: tromey

src/etc/gdb_providers.py

@@ -128,6 +128,9 @@ def children(self):
             self._data_ptr + index for index in xrange(self._length)
         )
 
+    def num_children(self):
+        return self._length
+
     @staticmethod
     def display_hint():
         return "array"
@@ -149,6 +152,9 @@ def children(self):
             self._data_ptr + index for index in xrange(self._length)
         )
 
+    def num_children(self):
+        return self._length
+
     @staticmethod
     def display_hint():
         return "array"
@@ -177,6 +183,9 @@ def children(self):
             for index in xrange(self._size)
         )
 
+    def num_children(self):
+        return self._size
+
     @staticmethod
     def display_hint():
         return "array"
@@ -478,5 +487,11 @@ def children(self):
             else:
                 yield "[{}]".format(index), element[ZERO_FIELD]
 
+    def num_children(self):
+        result = self._size
+        if self._show_values:
+            result *= 2
+        return result
+
     def display_hint(self):
         return "map" if self._show_values else "array"