monotonic clock: introduce duration type, split subscribe

We are introducing a `duration` type because it has a distinct meaning
from `instant`: an `instant` can only be compared to other `instant`s
from the exact same `monotonic-clock`, whereas a `duration` represents
a duration of time which can be compared to any other duration of time.

The `duration` type is motivated, in part, by a desire to reuse it to
specify durations such as timeouts in other WASI proposals.

Instead of taking a boolean specifying whether the u64 is an absolute or
relative time, `subscribe-instant` takes an `instant` type and
`subscribe-duration` takes a `duration` type.

wit/monotonic-clock.wit

@@ -11,22 +11,34 @@
 interface monotonic-clock {
     use wasi:io/poll.{pollable};
 
-    /// A timestamp in nanoseconds.
+    /// An instant in time, in nanoseconds. An instant is relative to an
+    /// unspecified initial value, and can only be compared to instances from
+    /// the same monotonic-clock.
     type instant = u64;
 
+    /// A duration of time, in nanoseconds.
+    type duration = u64;
+
     /// Read the current value of the clock.
     ///
     /// The clock is monotonic, therefore calling this function repeatedly will
     /// produce a sequence of non-decreasing values.
     now: func() -> instant;
 
-    /// Query the resolution of the clock.
-    resolution: func() -> instant;
+    /// Query the resolution of the clock. Returns the duration of time
+    /// corresponding to a clock tick.
+    resolution: func() -> duration;
 
-    /// Create a `pollable` which will resolve once the specified time has been
-    /// reached.
-    subscribe: func(
+    /// Create a `pollable` which will resolve once the specified instant
+    /// occured.
+    subscribe-instant: func(
         when: instant,
-        absolute: bool
+    ) -> pollable;
+
+    /// Create a `pollable` which will resolve once the given duration has
+    /// elapsed, starting at the time at which this function was called.
+    /// occured.
+    subscribe-duration: func(
+        when: duration,
     ) -> pollable;
 }