misc/wasm: pollute global JS namespace less

This commit changes wasm_exec.js so it only puts the single
name "go" into the global namespace. Other names became private
or were turned into a property/method of "go".

Change-Id: I633829dfd3c06936f092c0a14b9978bf855e41fe
Reviewed-on: https://go-review.googlesource.com/112980


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>