Change idom() method to get_a_dom(), update uses, and documentation.
Add support for more bytecodes.
Working on full opcode coverage and support for exception control flow.
Updates to llpython.opcode_util to better support loop and exception control flow.
* Inlcuded non-argument branching opcodes in the hasjump opcode list.
* Added all the SETUP_* opcodes to the hascbranch list.
* Modified itercode() to not output the EXTENDED_ARG bytecode.
* Cleaned up extendlabels(), and made it use the hasjump opcode list uniformly.
Initial attempt to support return statements in try-finally block in llpython.byte_control.  Added incomplete unit tests.