preamble_size: Include mitogen.core and clarify bootstrap size

After:
SSH command size: 759
Preamble (mitogen.core + econtext) size: 18227 (17.80KiB)

                        Original           Minimized           Compressed
mitogen.core         152218 148.7KiB  68437 66.8KiB 45.0%  18124 17.7KiB 11.9%
mitogen.parent        98853  96.5KiB  51103 49.9KiB 51.7%  12881 12.6KiB 13.0%
mitogen.fork           8445   8.2KiB   4139  4.0KiB 49.0%   1652  1.6KiB 19.6%
mitogen.ssh           10827  10.6KiB   6893  6.7KiB 63.7%   2099  2.0KiB 19.4%
mitogen.sudo          12089  11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB 18.6%
mitogen.select        12325  12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB  7.8%
mitogen.service       41581  40.6KiB  22398 21.9KiB 53.9%   5847  5.7KiB 14.1%
mitogen.fakessh       15767  15.4KiB   8149  8.0KiB 51.7%   2676  2.6KiB 17.0%
mitogen.master        55317  54.0KiB  28846 28.2KiB 52.1%   7528  7.4KiB 13.6%

preamble_size.py

@@ -8,6 +8,7 @@ import inspect
 import sys
 import zlib
 
+import mitogen.core
 import mitogen.fakessh
 import mitogen.fork
 import mitogen.master
@@ -35,7 +36,7 @@ conn = mitogen.ssh.Connection(options, router)
 conn.context = context
 
 print('SSH command size: %s' % (len(' '.join(conn.get_boot_command())),))
-print('Bootstrap (mitogen.core) size: %s (%.2fKiB)' % (
+print('Preamble (mitogen.core + econtext) size: %s (%.2fKiB)' % (
     len(conn.get_preamble()),
     len(conn.get_preamble()) / 1024.0,
 ))
@@ -49,6 +50,7 @@ if '--dump' in sys.argv:
 table = Table()
 print(table.header())
 for mod in (
+        mitogen.core,
         mitogen.parent,
         mitogen.fork,
         mitogen.ssh,