investigate the access witness issue

[gballet]

Context

When creating the proof, the values that find themselves in the access witness are wrong, they correspond to a state later than that of the post state. For example, in TestProcessStateless, a comparison of the first block's prestate with that of the access witness show:

key 6c99a3a0427cab63b7ab24f0683da88a1c5ed53f7b072b9e4efebd5dc412fd01 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key a365db4f33df4f95bf2ae41da5a1bc3c804c3e511e7fddff4eabd000b5c0d602 has correct value 0000000000000000000000000000000000000000000000000000000000000000
key 78fdaed8ac3619d9b9520a66b57fdc16a1ed03b29ab8e9fb021c39bf8cefdd01 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 78fdaed8ac3619d9b9520a66b57fdc16a1ed03b29ab8e9fb021c39bf8cefdd02 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key a365db4f33df4f95bf2ae41da5a1bc3c804c3e511e7fddff4eabd000b5c0d604 has value , should be 0000000000000000000000000000000000000000000000000000000000000000
key a365db4f33df4f95bf2ae41da5a1bc3c804c3e511e7fddff4eabd000b5c0d601 has value 0043fdd2679be00d000000000000000000000000000000000000000000000000, should be 000064a7b3b6e00d000000000000000000000000000000000000000000000000
key a365db4f33df4f95bf2ae41da5a1bc3c804c3e511e7fddff4eabd000b5c0d603 has value , should be c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470
key 78fdaed8ac3619d9b9520a66b57fdc16a1ed03b29ab8e9fb021c39bf8cefdd03 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 78fdaed8ac3619d9b9520a66b57fdc16a1ed03b29ab8e9fb021c39bf8cefdd00 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 6c99a3a0427cab63b7ab24f0683da88a1c5ed53f7b072b9e4efebd5dc412fd04 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 6c99a3a0427cab63b7ab24f0683da88a1c5ed53f7b072b9e4efebd5dc412fd03 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 6c99a3a0427cab63b7ab24f0683da88a1c5ed53f7b072b9e4efebd5dc412fd02 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 78fdaed8ac3619d9b9520a66b57fdc16a1ed03b29ab8e9fb021c39bf8cefdd04 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key 6c99a3a0427cab63b7ab24f0683da88a1c5ed53f7b072b9e4efebd5dc412fd00 has value 0000000000000000000000000000000000000000000000000000000000000000, should be 
key a365db4f33df4f95bf2ae41da5a1bc3c804c3e511e7fddff4eabd000b5c0d600 has correct value 0000000000000000000000000000000000000000000000000000000000000000

Analysis

There are several bugs here:

• some values are correct, but are not 32-byte aligned. This PR contains the fix for this;
• some values are initially nil, but since nil isn't recognized as a value, it is replaced with 0000000000000000000000000000000000000000000000000000000000000000
• some values, namely the balances, are wrong, for example the pre-allocated account should have a balance of 1000000000000000000 (000064a7b3b6e00d in little endian hex form), but the first value that is inserted is 0043fdd2679be00d, which is little-endian hex for 999969987500000000. That is an incorrect value, which probably comes from the fact that this is the sender account and that the sending costs is charged before the account witness is created.

[gballet]

Superseded by #83. Closing.