initialize R2 with data length

[ygrek]

It is convenient to know data size immediately, without going through indirect structure. What do you think?

[ygrek]

afaics the tests are failing because of smth unrelated :

======================================================================
ERROR: test_elf.test_datafiles('elf/wrong-version.data',)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/.local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/build/iovisor/ubpf/test_framework/test_elf.py", line 176, in check_datafile
    elf = generate_elf(data['pyelf'])
  File "/home/travis/build/iovisor/ubpf/test_framework/test_elf.py", line 163, in generate_elf
    return serialize(parts)
  File "/home/travis/build/iovisor/ubpf/test_framework/test_elf.py", line 146, in serialize
    serializer = s.Elf_Ehdr.build
AttributeError: 'ELFStructs' object has no attribute 'Elf_Ehdr'

[pchaigno]

#24 should fix the tests.

Could you add a test case for this change?

[coveralls]

Coverage increased (+0.01%) to 96.694% when pulling b7caed9 on ygrek:r2 into 10e0a45 on iovisor:master.

[pchaigno]

@ygrek Could you add a test for this change please? I won't merge without it. I can help write the test if necessary of course.

[ygrek]

(merged)