Include Debugger

Add debugger modeled after DOS' debug.com.

Also includes minor fixes/improvements:

• General assembler improvements: shorter encodings / more operands supported
• Gain 2 clusters (faulty interpretation of max valid cluster)
• Print free disk space on "DIR"
• Open files are closed on process exit
• Limited support for Int21/AX=4B01 and termination handlers (as needed by debugger)

v1.0 Initial Release

Initial Release