Go 1.13 introduced two more low-level fields that would've been
referenced by essentially dead code, so I decided to replace original
pool.go entirely. As a byproduct, this might yield a tiny decrease in
output size (assuming DCE hasn't been pruning the unexported methods
away already).

math.FMA() implementation introduced a non-exported function zero(),
which conflicted with out own variable. Resolved the conflict by
renaming our variable.

Original commit by @visualfc: goplusjs/gopherjs@ad4d281

Looks like a few new functions were added to the package, with a few new
constants. Neither of them are really relevant in the JS context and
arguably we shouldn't be compiling this code in the first place (see
issue gopherjs#991), but this lets the package compile for now.

I'm not sure of the role of the seq field plays outside of the time
package, but I'll add it there. In the upstream code the runtimeTimer
struct seems to replicate (and require to be compatible with)
runtime.timer struct, however in GopherJS it is clearly not and it
doesn't seem to be a problem.

Similarly, in the upstream startTimer, stopTimer, resetTimer and
modTimer are actually implemented in the `runtime` package, but since we
already re-implemented two of them here, I'll just keep this package
self-contained.

Co-authored-by: Jonathan Hall <flimzy@flimzy.com>