This change adds tests for the virtual clock and aligns the interface
with the time package by renaming Cancel to Stop. It also removes the
binary search from Stop because it complicates the code unnecessarily.

core/state: optimize some internals during encoding

Gollvm has very aggressive dead code elimination that completely
removes one of these two benchmarks.  To prevent this, use the
result of the benchmark (a boolean), and to be "fair", make the
transformation to both benchmarks.

To be reliably assured of not removing the code, "use" means
assigning to an exported global.  Non-exported globals and
//go:noinline functions are possibly subject to this optimization.