improve x86 emulation so it can handle final programs up through lambda

[AndrewTolmach]

A few fixes and improvements:

• With appropriate flag setting, emulator checks that callee-save registers are not disturbed by calls and garbages caller-save registers. This allows easier debugging.
• Add additional support to allow emulation of unified final programs (although still not lists of FunctionDefs) up through lambdas. The remaining chapters still require additional primitive functions.