naming cleanup (pxll, BOX_INTEGER, etc...)

[samrushing]

There are several naming issues that have accumulated over the years.

• s/pxll/irken
• s/BOX_INTEGER/TAG_INT
• lib/pair.scm -> lib/list.scm
• LIST/PUSH -> list/push

These need to be done in one big change.

[samrushing]

Not quite finished yet... there's still box()/unbox() in irken.h and all over the llvm backend.