Gtirb-test-helpers

- Removed additional_entries from add_function
- Added support for Python typing
- Exposed add_section to __init__ so it can be imported

- add_code_block and add_data_block now accept (int, int) tuples as keys for
symbolic expressions, with the first item being the offset and the second
item being the size (which gets put in the symbolicExpressionSizes aux data
table)
- add_function now accepts a set of entry blocks for the entry_block parameter

- Added additional_entries parameter to add_function for specifying multiple
entry points to functions

- Added byte_order to create_test_module and defaulted to inferring it from
the module's ISA

- Initial version