Add specs for rb_time_* C-API functions

Adds specs for rb_time_num_new, rb_time_interval, rb_time_timeval and
rb_time_timespec

See issue #2713
Add rb_time_* functions to C-API

This adds rb_time_num_new, rb_time_interval, rb_time_timeval and
rb_time_timespec.

Fixes #2713