fix stackoverflow in calldataload, codecopy, extcodecopy + some tests
avoid code repetition in vm
minor fix
even less code
Merge remote-tracking branch 'upstream/develop' into bugfix

Conflicts:
	libevm/VM.h
set first 12 bytes to zero in ecrecover
Merge pull request #600 from CJentzsch/bugfix

set first 12 bytes to zero in ecrecover