Currently the block stack size is hardcoded to 20.  Similar to how the value stack is dynamically sizable, we should make the block stack dynamically sizable.  This will reduce space on average (since the typical number of blocks for a function is well below 20) and allow code generators to generate code with more deep nesting.  Note: the motivation is not necessarily to reduce memory usage, but to make L1 cache misses less likely for stack objects.