Backend support for parser hierarchy widget

- left hand side shows the parser about to be invoked (top of current parser stack), as well as its address
- right hand side shows the current position in the input + the next 32 bytes
- step button executes hammer-parse-step 1

Start the GUI by sourcing gui.py after parser-name-instrumentation-gdb.py and utility-commands.py have been loaded

- use ParserStack.p_stack directly, instead of the ParserStack object
- HDoParseBreakpoint looks at the "index" field of the parse state, not "pos"

- Trying to access fields of 'state' generated an exception
- stop_pos >= pos should have been stop_pos <= pos

Although the command runs, invoking "continue" without an inferior running
produces an error message with no other feedback