[C++] Implement (safe and unsafe) casts between timestamps and times of different units

[asfimport]

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Type casting function kernel suite (Is contained by)

PRs and other links:

• GitHub Pull Request #1245

_{Note: This issue was originally created as ARROW-1484. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Resolved by PR 54d5c81