allocator: add 16kb page bump allocator

This is a private allocator that can be used for large groups of
allocations that are tied to a specific object.

For example, the callgraph owning many nodes can use this so all the nodes
are allocated together, but also freed in a single stage ignoring all the
complex GList linked nodes among them.

src/libsysprof/meson.build

@@ -131,6 +131,7 @@ libsysprof_public_headers = [
 libsysprof_private_sources = [
   'mapped-ring-buffer-source.c',
   'sysprof-address-layout.c',
+  'sysprof-allocator.c',
   'sysprof-controlfd-instrument.c',
   'sysprof-descendants-model.c',
   'sysprof-document-bitset-index.c',