Allow more integer types when creating Instants

[crawford]

This allows any type that can be converted into an i64 to be used when
creating an Instant. Because this is no longer a concrete type, this
change may break existing code like the following:

error: attempt to multiply with overflow
   --> src/time.rs:282:37
    |
282 |         epoc = Instant::from_millis(2085955200 * 1000).into();
    |                                     ^^^^^^^^^^^^^^^^^
    |
    = note: #[deny(const_err)] on by default

[whitequark]

You've demonstrated the downside of this PR (thanks!) but what are the concrete upsides?

[crawford]

Instead of writing:

let timestamp = Instant::from_millis(rtc.cnt.read().cnt().bits().into());

I'll be able to write:

let timestamp = Instant::from_millis(rtc.cnt.read().cnt().bits());

It seems more common in the larger Rust ecosystem to perform the conversion in the function than at the callsite.

[whitequark]

OK. It seems unlikely that there is a lot of code creating Instants from literals, and the fix is anyhow trivial.

[whitequark]

@m-labs-homu r+

[m-labs-homu]

📌 Commit bd63e4e has been approved by whitequark

[m-labs-homu]

⌛ Testing commit bd63e4e with merge d381dcd...

[m-labs-homu]

☀️ Test successful - status-travis
Approved by: whitequark
Pushing d381dcd to master...