Implement a builder API for Now

[nekevss]

This PR implements a builder API for now and closes #249.

It has some notable changes to the approach to Now.

Mainly Now is essentially cannot be built without using the new NowBuilder and any method called on Now consumes it.

For general purpose, a Temporal namespace struct was created with now and try_now methods, which uses the NowBuilder to construct a Now with values from the system.

This does differ a bit from the specification in that we are eagerly calling the host system time zone, rather than lazily when the user time zone is None. But I think this can still be worked around on the engine side if lazily calling is preferred and is more a place where the native Rust API will differ from the engine implementation. The work around to this is a trait based approach that is available to see on the now-builder branch, but I think this a simpler and cleaner approach than leaning on traits.

List of changes:

• Now updated and NowBuilder added.
• Feature flagged Temporal struct
• time module renamed to unix_time

[HalidOdat]

Looks good to me! :)