[rust] Update Time and Duration methods to the into_* form.

It is more idiomatic and avoids ambiguity as to whether e.g. `nanos`,
`micros`, and `millis` refers to the subsecond part of a struct or
the entire value represented in that unit of measure.

Updates all uses of the methods in fuchsia.git and deprecates the old
methods.

TEST=CQ

Change-Id: Ibe67ffd028169cffdac0c1ce662bdb78fe7b1cd4