WebAssembly javascript build of PARI

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

I have added a third experimental, version of the javascript version of
PARI/GP which relies on WebAssembly (see <http://webassembly.org/>):

<https://pari.math.u-bordeaux.fr/gpexpwasm.html>

I have not included compat codes for browsers that do not support
WebAssembly yet. So if it works for you, then you are really using
WebAssembly. Otherwise you can still use the other versions

I have also rebuilt the asm.js version with a different option which
should make it faster.

Cheers,
Bill.