Hera-py

Fixed
- The assembler no longer produces code for debugging instructions like `print_reg`.
- `ASR` now rounds towards negative infinity for negative numbers instead of zero.
- `DEC` and `SUB` now set the carry flag correctly in certain edge cases.

Added
- The command output includes the current version of the HERA language.

Added
- Start-up message for debugger with instructions for new users.
- Entering a blank line in the debugger repeats the previous command, like in `gdb`.

Fixed
- Runtime errors on Python 3.7 due to use of typing library.

Added
- The `doc` debugging command, for viewing documentation on individual HERA operations.
- The parser now understands C-style `ifdef` and `ifndef` macros, with `HERA_PY` as the only defined symbol.
- The register calling convention version of the Tiger standard library function `chr`
- The `--init` command-line option for initializing registers.
- New `--throttle=<n>` syntax for the `--throttle` option.

Fixed
- The register calling convention version of the Tiger standard library function `getchar`, previously broken, has been fixed.

Added
- The `OPCODE` instruction.
- The `asm` and `dis` debugging commands, for assembling and disassembling HERA code in the debugger.

Changed
- The readline library is no longer required for `hera` to run.