time: Eagerly bind delays to timer (#1800)

## Motivation

Similar to #1666, it is no longer necessary to lazily register delays with the
executions default timer. All delays are expected to be created from within a
runtime, and should panic if not done so.

## Solution

`tokio::time` now assumes there to be a `CURRENT_TIMER` set when creating a
delay; this can be assumed if called within a tokio runtime. If there is no
current timer, the application will panic with a "no current timer" message.

## Follow-up

Similar to #1666, `HandlePriv` can probably be removed, but this mainly prepares
for 0.2 API changes. Because it is not in the public API, this can be done in a
following change.

Signed-off-by: Kevin Leimkuhler <kleimkuhler@icloud.com>

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