Add `teprintf()` function

A new function to print to stderr prefixed with a timestamp. The idea is
to make the printed line's format match the Rust logger's.

The Rust logger prints timestamps like this:

    2018-11-04T03:16:31.938+01:00

`teprintf` does:

    2018-11-04T15:02:21+0100

Unfortunately, I wasn't able to reproduce it exactly. It's missing the
milliseconds because I didn't want to deal with using `gettimeofday`,
and the time zone format isn't right because I didn't know how to format
it easily with a colon.

Resources:
Getting the time in C:
https://stackoverflow.com/questions/9596945/how-to-get-appropriate-timestamp-in-c-for-logs#9596994

Variadic functions in C:
https://stackoverflow.com/questions/205529/passing-variable-number-of-arguments-around

Getting milliseconds in C:
https://stackoverflow.com/questions/10192903/time-in-milliseconds#10192994
https://stackoverflow.com/questions/3673226/how-to-print-time-in-format-2009-08-10-181754-811
https://stackoverflow.com/questions/361363/how-to-measure-time-in-milliseconds-using-ansi-c

Concatenating strings in C:
https://stackoverflow.com/questions/308695/how-do-i-concatenate-const-literal-strings-in-c#308712
https://stackoverflow.com/questions/8465006/how-do-i-concatenate-two-strings-in-c

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