Fix dex module inconsistencies

[vthib]

Fix several inconsistencies in the dex module:

• some bytestring values were not properly set: the magic constants, header.magic and header.signature. Those were cut if a nul byte was present (which is the case in the magic values

• dex.field[*].static and dex.field[*].instance were defined, but not declared, making their use impossible. They are now properly declared.

• several fields in dex.method[*].code_item were declared but never defined: padding, tries and handlers. Those are removed since they couldn't have been used.

[vthib]

I'm not sure what the macosx build issue is. It complains about missing libjansson, which is not related to the dex module but to the cuckoo one, which i do not touch. So afaict it is unrelated.