Create a register value object.
DATA can be any value.
PRINT-FUNC if provided controls how `list-registers' and
`view-register' print the register. It should be a function
receiving one argument DATA and print text that completes
this sentence:
Register X contains [TEXT PRINTED BY PRINT-FUNC]
JUMP-FUNC if provided, controls how `jump-to-register' jumps to the register.
INSERT-FUNC if provided, controls how `insert-register' insert the register.
They both receive DATA as argument.
(fn DATA &key PRINT-FUNC JUMP-FUNC INSERT-FUNC)