gstreamer: Add seconds_f32 & seconds_f64 methods and from_seconds_f32 & from_seconds_f64 constructor for ClockTime and Signed<ClockTime>
Closes #443 (closed)
Most of the test cases are copied from std.
I realized that std Duration does not have try_from_XXXX
variants for other constructors (only on the float ones), so I think we're good without them.
They are also non-const since the used Duration methods are also non-const, and float arithmetic on const functions are not supported in stable.
Edited by Dave Patrick Caberto