debug: add interactive debug mode

[jow-]

Introduce a new interactive debug cli which can be started by invoking the ucode interpreter with the -x flag.

The main debugger functionality resides in the debug library while the cli frontend just gains some minimal amount of code to load and instantiate the debug library when the -x option is passed.

As a precondition for the new debug functionality, the ucode VM has been extended with low level breakpoint support.