More millisecond procedures

[Lassi Kortela]

> Attached is a set of two patches.  The first one simply adds a
> deprecation notice to current-milliseconds and adds the new procedure
> current-process-milliseconds.

Racket also has `current-inexact-milliseconds`: "Returns the current 
time in milliseconds since midnight UTC, January 1, 1970. The result may 
contain fractions of a millisecond." 
<https://docs.racket-lang.org/reference/time.html>

In general, millisecond precision is probably attainable on most/all 
operating systems without too much fuss. It's the sub-millisecond range 
where the problems begin.

Would it make sense to write a really simple SRFI about these 
millisecond-precision procedures?