This commit improves wasm_exec.js to give more control to the
code that uses this helper:
- Allow to load and run more than one Go program at the same time.
- Move WebAssembly.instantiate out of wasm_exec.js so the caller
  can optimize for load-time performance, e.g. by using
  instantiateStreaming.
- Allow caller to provide argv, env and exit callback.

Updates #18892

Change-Id: Ib582e6f43848c0118ea5c89f2e24b371c45c2050
Reviewed-on: https://go-review.googlesource.com/113515Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>