Gtirb-rewriting

- Fixed an issue where running rewriting over leaf functions multiple times
could cause the red zone to be clobbered on 64-bit ELF
- Updated to new ARM symbolic expression types
- Switched to capstone-gt

- Added support for symbolic expressions in data.
- Fixed CallPatch redundantly aligning the stack
- Made CallPatch default to aligning the stack and preserving caller-saved
registers. Clients can opt out of this by specifying align_stack=False and
preserve_caller_saved_registers=False to CallPatch.

- Added support for ARM64
- Exposed the Assembler class
- Added ABI.temporary_label_prefix and InsertionContext.temporary_label

- Added support for the .byte directive
- Added support for printing data blocks with show_block_asm

- Added support for symbolic expressions with a constant addend

- Exposed the ABI class, to allow transforms to get information about the
target
- show_block_asm now prints out symbolic expressions