Removes context parameter from instruction-scoped operations (#923)

We originally had a `context.Context` for anything that might be
traced, but it turned out to be only useful for lifecycle and host functions.

For instruction-scoped aspects like memory updates, a context parameter is too
fine-grained and also invisible in practice. For example, most users will use
the compiler engine, and its memory, global or table access will never use go's
context.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

sys/clock.go

@@ -1,7 +1,5 @@
 package sys
 
-import "context"
-
 // ClockResolution is a positive granularity of clock precision in
 // nanoseconds. For example, if the resolution is 1us, this returns 1000.
 //
@@ -11,14 +9,14 @@ import "context"
 type ClockResolution uint32
 
 // Walltime returns the current time in epoch seconds with a nanosecond fraction.
-type Walltime func(context.Context) (sec int64, nsec int32)
+type Walltime func() (sec int64, nsec int32)
 
 // Nanotime returns nanoseconds since an arbitrary start point, used to measure
 // elapsed time. This is sometimes referred to as a tick or monotonic time.
 //
 // Note: There are no constraints on the value return except that it
 // increments. For example, -1 is a valid if the next value is >= 0.
-type Nanotime func(context.Context) int64
+type Nanotime func() int64
 
 // Nanosleep puts the current goroutine to sleep for at least ns nanoseconds.
-type Nanosleep func(ctx context.Context, ns int64)
+type Nanosleep func(ns int64)