libsysprof-analyze: add bitset index

This is meant to be a simple index helper where we already have a bitset
created for a low-cardinality index. We can reuse the read-only bitset
and filter on the document itself.

This index will keep a reference on the document so it's lifetime persists.

src/libsysprof-analyze/meson.build

@@ -21,6 +21,7 @@ libsysprof_analyze_public_sources = [
 ]
 
 libsysprof_analyze_private_sources = [
+  'sysprof-document-bitset-index.c',
   'sysprof-mapped-file.c',
   'sysprof-memory-map.c',
   'sysprof-mount-device.c',