shower thought: don't store the code in the tree

[gballet]

Since it's pretty obvious that a lot of space can be saved by not storing empty leaves, it is worth trying not to store the leaves containing only code. This means:

• In account headers, only store the account leaves and data
• In regular code groups, only store the commitments.

This works in geth, because the code is stored out-of-band and with a better storage format.