PATCH for gob2 2.0.11 bugfix & added feature

This patch adds support for a new keyword 'prealloc' which allows the
programmer to specify a value for the n_preallocs field in GType,
defaults to 0 if not specified. It also fixes support for bitfields.

test code:

requires 2.0.11
prealloc 90

class :test from G:Object {
public int v1;
public int v2:1=0;
}