bug#26163: time-difference doesn't detect error of differing time types

[Zefram]

scheme@(guile-user)> (use-modules (srfi srfi-19))
scheme@(guile-user)> (time-difference (make-time time-tai 0 1) (make-time 
time-utc 0 1))
$1 = #<time type: time-duration nanosecond: 0 second: 0>

SRFI-19 is explicit that it "is an error" if the arguments to
time-difference are of different time types, and correspondingly the
Guile documentation says the arguments "must be" of the same type.
It would be very easy for time-difference to detect and signal this error.
It's not absolutely a bug that it currently doesn't, but it would be a
useful improvement if it did.

-zefram