Add leaf group "erasure" function

[gballet]

This is needed to fix tests in rjl493456442/go-ethereum#12 : Gary's stuff will call DeleteAccount if the account is non-existent. While this is quite unclean and should be fixed at the geth layer, I reckon that it is much more difficult to do than designing the verkle library to handle this.

As a result, I introduce an EraseStem method, which is used to replace InsertAtStem when deleting a function: it will go through the tree, find a group if it exsits, and overwrite all values present in that group. This is less invasive than the current method of calling InsertAtStem with 256 non-zero values.

It has the following drawbacks:

• This behavior needs to be specified in the spec, since nil values are not overwritten
• It also breaks the replay code, which uses the trick of checking that all values are 0 in order to simulate the "pre-verge" behavior of account deletion.

[gballet]

Fixes #448