[dev-context] Time primitives

[Taco Hoekwater]

Hi Hans,

Would new (pdf)etex primitives like these help benchmarking context?

Two static read-only values that amend the normal TeX ones like
\time and \year, giving the passed seconds since 1 jan 1970 at
start-of-run:

   \epochseconds  % integer part
   \microseconds  % fractional part (between and 0-1000000)

And one dynamic one:

   \microinterval % interval since start-of-run, in microseconds

The static seconds and microseconds have be split in two because the
multiplication of two C long-s cannot fit in one \count register.

I need the initialization values only so that I can create
meaningful intervals, but I thought it might be useful to have
them available.

If you prefer something more or different, let me know.

Taco