Move fs::Metadata::modified to modified_as_ms and reserve for the real Datetime type

[m13253]

As is documented here: std::fs::Metadata::modified, method modifed returns the modification time in milliseconds since the epoch.

We don't yet have a Datetime yet. We even haven't decided which epoch to use at the moment! But we will surely have one some time in the future.

I suggest we move modified to modified_as_ms and reserve for the real Datetime type.

My reasons:

1. If we don't do that, inconsistency will be introduced when the real Datetime type is invented.
2. Some filesystems (e.g. ext4) provide modification time in a nanosecond accuracy (10^-9s). Milliseconds are far not enough, a single u64 is far not enough.
3. Reserving in advance will reduce the pain at the transition. Instead of making old codes fail to compile, we can issue a "deprecated" warning then, to let the programmer switch to the final real modified.

Related PR #739

[alexcrichton]

Thanks for the issue! This will definitely come up in stabilizing Metadata::modified, and as-is it is definitely not slated for stabilization (for the exact reasons you specify). As a result I'm going to close this issue as we probably don't want to open an RFC issue for all unstable APIs!