Command syntax improvements

The hammer-parse- and hammer-parser- distinction is getting unwieldy with more features.

Opening this issue for collecting ideas, observations, designs for an improved interface. GDB's subcommand feature should come in handy. For example:

hammer backtrace

hammer step
hammer finish
hammer apply

hammer parser name <arg> # Could be turned into an argument as well
hammer parser <addr>

hammer input preview
# ...